I bought a Sony Xperia X10 in April, really liked it. My mate then upgraded from an iPhone to a Samsung Galaxy S on my advice due to what I'd heard/read on various sites. The moment I saw his phone and used it a bit my heart sank. I'd bought the wrong phone.
I subsequently bought a Galaxy S and am very happy with it. I think it's the best android phone on the market. I had a tricky decision deciding between the X10 and Desire initially, but had the Galaxy S been on sale when I first got Android it would've been a no brainer.
I'm sure most on here would agree it's probably the best phone available in the UK at the moment. It does have one draw back, it has no camera flash. Which may be an issue for some who require some night photos.
I'd like to think that opinion is unbiased as I had to choose between an X10 and a Desire initially, then was so impressed by the Galaxy I felt the need to buy it less than 3 months after getting the X10.
Anyway, limpid's advice is sound, have a look at both phones in a store and see which one you take too. But under the hood, the galaxy s has the better hardware in every department.