It's a tricky question to answer without knowing more. Given a choice of top agents in either country, which would you go for?
There is no harm submitting a batch of queries to both countries, testing the water, and seeing what comes back.
I am in a similar position. I'm an Irish writer, and initially queried only UK agents. Then I realised that my story might have a better chance of getting published in the US (and some feedback indicated my style was quite American - whatever that means). So I am querying in both markets now, and the response has been a lot more favourable in the US. I am still querying in the UK, but the focus is more on the US.
Before querying in the UK you should be aware of some differences (copying this from what I wrote in another thread):
* Some agents do not accept simultaneous submissions, even at query stage. I think Maeve Binchy's agent is one of these. So if she is your dream agent, keep this in mind.
* A LOT of the big agencies only accept snail mail submission.
* My personal experience is that they are slower to respond.
* Many will only accept submissions from UK (and Irish) based writers.
Re. the query letter/cover letter: It varies from agency to agency whether they want a "cover letter" like the one shown above or a more American-style query letter. Generally this information is on their website (if they have one) or in the Writers & Artists Yearbook http://www.writersandartists.co.uk/
(a must-purchase if you are submitting to the UK). If they don't mention it, a good rule of thumb is: electronic submission - query letter; snail mail submission - cover letter.
If it's a cover letter, it's less about the book, and more about you, the genesis of your idea and your background (kind of like an extended bio-section). If it's a query letter, it's the American-style query letter you are familiar with.
With a cover letter submission, a lot of weight is put on the synopsis (more so, I would guess, than American agents may put on it). I have heard many UK agents comment that the cover letter is barely read, and the synopsis decides for them whether they go any further (read the submitted pages/request pages).
Also, with a cover letter submission, standard is a synopsis plus the first three chapters/first fifty pages, although this too varies from agency to agency.
If you have any more specific questions, just post em up here or PM me and I will be happy to help.