Which role do you find is more challenging, acquiring new clients or maintaining excellent relationship with existing customers ?

This is an excellent question, and my take is that the answer depends on the profile of the sales representative more than anything else.
Acquiring new customers requires skills/behaviors to enjoy cold calling, meeting new faces, closing business with a company that never dealt with you.
Proper "hunters" have these skills, they enjoy running after new, new, new.
Developing existing relationship is a completely different game.
It requires skills/behaviors such as communication, patience, potentially a passion for the client's business, and the capability to deal with problems encountered by the client.
"Farmers/Gatherers" have these skills, they are good at building trust for a longstanding relationship.
Both roles are equally challenging, success or failure comes from assigning the right sales representative to the right position.
By the way, being a "Hunter" or a "Farmer" is not an attribute for a whole career.
I worked together with many sales representatives who successfully converted from one profile to the other (either way) and adapted themselves to the new context.
Best regards Bernard

Actually both are challenging but for me i think maintaining the relationship is more difficult, you can close a deal by simply give a lower price or by personal connection or taking advantage of being the NEW company or supplier that they want to try.
After getting the lead and start your relation with the clients you actually begin the real objection handling simply because you have more to handle, your service or product quality, delivery time, response time, after sales service...etc.
In my opinion your client should be your own sales support after they deal with you, you should be able to generate the maximum leads through their referrals and he should be comfortable mentioning you (by name) and your service to other potential clients.
The trick is to make them help you without asking :) Sometimes your company name or brand close the deal for you but it totally depends on you to maintain, develop and make the most out of your clients.

It really depends on the market and the product type, is it a price-sensitive market? or a semi-monopoly environment? on a generic view, I believe the acquisition of new clients is more challenging specially when the product is a new technology and the client is not aware of this trend.
maintaining excellent relationship with existing customers is more based on non-sales personnel than sales personnel, it is sensitive to service level and quality, the salesperson personality, mentality and personal performance is only a small role to play to "recover" problems created by service department.

I am the type of Sales Professional that enjoys both Acquisition and Maintenance of Customers.
The question raised is a excellent one and can only be answered by understanding business.
You have identified the only 2 components of business that actually matter.
(If you want to know more on this subject let me know).
Creating and Maintaining Customers: Creating customers is the easy part as they are only usually dealing with one or two people within your company and if you have a quality product that you can present the benefits to your potential customers then gaining them will be easy.
However you need to present the customers a Total Cost Solution that shows you are actually taking an interest in your customers business, because they only want to do business with people who care about them.
In my experience it is much more difficult to maintain customers as there are now many different facets of the business that the customer is dealing with.
These facets include, after sales service departments, parts departments, actual service technicians, etc.
My experience has taught me one major component that can overcome any issues that the customer may have: COMMUNICATION.
Yes communication is the key to customer maintenance, this is above all else including product quality, service timeliness, product delivery, etc So if you ask your customers what they really want they will all tell you the same: Effective Communication because all they want is to be informed about what is going on, so if need be they can make contingency plans or you can work through the issue together.
I have many experiences throughout my Sales Career that will back this up.
Kind Regards Simon Johnson

Both of them are challenges, required and necessary in area of sales to keep business running vertically and horizontally.

it is completely depending on the kind of business and product you deal with...
in my 5 years experience in financial services what i experienced is that ....if you maintain excellent relationship with your existing customers,they will surely give you new customers...!!

Which role you find more challenging, acquring new clients or maintaining excellent relationship with existing customers ? Where is the largest challenge?Hard to tell! I tend to believe that bringing new customers to the company within the specific budget-depending the company,product and generally the sales situation- is little more challenging.
It brings more adrenaline! Maybe more stressfull process!! More psychological stamina is needed.
It makes you use little more of your senses and mind ...Are you ready to handle continuous "no Answers" till you come up with the one exhilarating "Yes" answer ! I perceive that most people would go for the Farmer role since, to some extent, can be a less demanding role(in terms of psychological stamina).
However, i do not mean that keeping and developing a customer is free of stress and challenge.
Both roles though are equally important because a company as a live organisation needs both new and old blood to stay alive.
Generally speaking, the salesperson should have developed equally the traits of the "Farmer" and "Hunter" in order to achieve optimum results in the long run.

It is relatively easy to find a new client, but a great challenge in maintaining the existing.
Both are necessary for any business.
But I still believe in and follow in the80-20 rule.80% of your business comes from20% of your customers.
Hence maintaining relationships is tough if you do not understand your client and appreciate his challenges and assist him with timely solutions.
Your existing customer has access to all departments in your organization, hence it is imperative that your company ensures identical customer response and treatment across the board.
this itself is a big challenge.
It is also necessary to identify the growth your customers are going through hence you need to make sure your capabilities are on par with that growth else your customers will have to start looking at your competition due to your shortcomings and failures.
lastly, never forget one unhappy customer will prevent10 prospective customers from coming to you.

Simply answering the below three question will provide with answer for mentioned above question : 1-How much of your target has been achieved for ( month-quarter-year ) 2-Is your current clients providing you with target you need.
3-Does your product need you to acquire new clients.