I've went through several different screen protectors. If I'm going to have one, I want one that doesn't acquire fingerprints. That's my main reason for having one as I never had one on my 1st gen and got not one scratch on my phone. The screen is pretty tough and I am always careful with my phones.
Either way - the 2 that I've found to be the best are the Crystal Shields or the Anti-Glare matte ones the Apple store has.
The crystal shield has the same feel as the actual screen, and the matte one has a little bit more of a smooth feel, hard to explain, but kind of like a frosted look. I liked both a lot.
I've used mirror screen protectors and ones of eBay but they never seem to last long. The corners bubble or they start to peel.
The mirror ones acquire fingerprints just as much as the regular screen does, so to me, there's no point. I liked the matte ones because it NEVER acquired fingerprints. I may pick up one of those to put on the new ones if me and the g/f spring for those when they come out. If not, I'll pick up some more for our current 3Gs.