Both are excellent choices - you won't regret either one. The Ruger is extremely well built, but I'm inclined to think you need to drop in a Volquartsen trigger, sear, and extractor to really get top performance out of it (The whole mess will run you something like $60, but you can do the work yourself if you're at all mechanically inclined). Once you put in those part, hooooo boy do you have a honey of a shooter. It's possible to get a specimen that's finicky with ammo, but usually the extractor will fix it. If not, there's a magazine mod that can be done to help the ammo feed. I haven't heard of one having problems with anything between those two fixes, and that's shooting the nastiest, dirtiest, cheapest ammo available. Ruger customer service is better than superb if you ever have a problem (or desire, really). They take care of virtually anything, typically on their dime. It's really, really good.
The Buck Mark is probably better out of the box, but short of flipping the spring, I'm not aware of anything you can do to "upgrade" it. The base model is prone to cracking the rear sight (It's plastic and you can overtighen the screw). You'll pay a bit more for the Buck too, but if you're dropping the money for the Ruger upgrades I'm sure its comprable.
I chose the Ruger, twice now. The Buck felt extremely nice in the hand, but I didn't like the base model and didn't want to spend the extra $200 for the URX upgrade. The Mark III I bought new, threw the Volquartsen stuff into, and am absolutely in love with. It's so good its disgusting. I bought a Mark II because I was so impressed with it and needed a spare for when I brought guests shooting, and it's made me nothing but happy too.
Either is fine though.