Which do you think is better: hiring an marketing and communications agency to promote your company/brand or keeping it in-house?

This is a more company related needs. In house marketing staff will remain in a bubble of his own knowledge in agreement to the SOP of the firm while an external agent will bring in new ideas which has been tried, tested to achieve success: further filtration will also be carried out to know the lapses of the firm  thus suggesting every possible remedy and not tuned under the umbrella of the firms' policy. On the contrary, in house staffs can be self thought or sponsored by the firm to get more educated in the vast world of marketing. 

As mentioned earlier it all narrows down to WHAT  and HOW a firm need to  delve into the world of marketing to be a step ahead of their game

I think both options will escalate companies brand identity. But if you think about cost and time in-house team is a must!  

There is two different type of work please find the below points..

Agency promote your brand:

1. Agency will do the job but it will share work and they charge full amount for promotion.
2. Agency if dedicate the person they will charge you more amount because one person dedicate only your brand promo in the market.Agency charge you more amount, but in other hand you are save for more hassle of employ and his benefits. 
3. Company benefit they gave agency target time to they will doing the promotion on targeted time line.

Company Benefit if Hiring the Marketing Team:

1. Company have ton over they bear the marketing team it is better to take their own team to get facilitate in company benefits.
2. Company have their own team to complete the project.