which is better, specialization of one field of translation; legal, or working on all fields?

In my opinion it depends on what kind of work you are looking forward to, for example, as part of my work as a translator for the CNN and other news agencies, i've done translation in many different fields, from politics to agriculture to medicine...etc I advice to go for all, for once you you have a new word in your dictionary you are rated as being more resourceful.
And at the end of the day, the more translation you do, the more you get paid.
Good Luck with your choice

You need, and I think you can work in all fields.
As a translator you can never guess how would be the nature of the next context? Most of the content of a legal contract could be describing technical or engineering specifications.
However, if you are a law graduate, I prefer you concentrate on legal, because companies need them as bilingual legal advisors.

I think that "better" is decided by the person who intends so, yes I agree that legal translation needs the translator to be specialized, or translations are needed too when the quality is the key translation feature

Well...
being a specialized translator in a certain field requires that you dedicate all your time, energy, and research into learning all there is to learn about your area of expertise in translation...
However what if after you feel like you have become highly qualified as a translator in a specific domain and would like to turn your attention to a new field and give it all your time and efforts ? Going on this way what if one can build up his knowledge in all the common areas of translation, wouldn't that make you a specialist in general translation ?!!! I don't know, maybe it's a young translators dream ! It sure would be legendary to achieve...

Sure that depends on your kind of charisma .
Are you kind of people who like to learn something new everyday ? Or do you prefer daily routine? .
But in both cases , you will get new experience .
Legal translation is totally different from common translation .
It complies with special grammars.
For example , we do not use( must ), for commitment in legal translation .
We use shall and should.
In both case , there are unspectacular experience .

of course it is better to specialize in one field

I think specialization of one field of translation not important you can take training courses in many field this will help you to find good job.