I am old school and I prefer a proven track record. I have 4 generation 3 Glocks. I saw a Gen 4 G26 but I'm not into the newer design. I'm holding out for a Gen 3.
I think that is why Glock is still selling the Gen 3 side by side with the 4's. There are a lot Glock fans that become partial to a specific model (RTFs for example).
Personally I think that the grips on the Gen 4 are much more comfortable but since I have only owned Gen 4s and have a lot more experience with them my opinion is heavily biased. I also have more than 4000 rounds through my Gen 4 G19 without a problem (or having to replace a single part) and so have no worries at all about reliability.
I know quite a few Gen 3 owners who have upgraded happily to Gen 4s and then I know quite a few that are happy staying with their Gen 3s. I don't think you can go wrong either way.
As for me I would not buy a brand new Gen 3 over a Gen 4. Now I would happily take a free Gen 3 or pick one up if the price was really, really good...