Generally you'll find a 30-40fps drop in velocity with 0.25g over 0.20g, but the benefits far outweigh the drawbacks. Fps isn't as important as one might think, aka. higher velocity isn't gonna get you more kills necessarily. Sure, one might think it's harder to dodge a lighter/faster BB than a heavier/slower one, but those only really happen at longer ranges (100ft+) in the open when the target sees you shoot at them and moves. Since the higher fps acheived with 0.20g BBs will get them to your target faster, they will have a larger spread by the time they get there due to their lighter weight (air resistance). 0.25g will stay on course in a tighter spread farther out.
And as far as range goes, I've heard some people claim that 0.20g BBs will have a farther range than 0.25g. This is true, but you have to look at what kind of range you are talking about. 1/ If maximum range is where the BB will evenutally land, then yes, a 0.20g BB might very well do that. 2/ If one is talking about effective range, meaning percentage of hits on a target 100+ft away, then 0.25g will have it in spades.
Anyone remember this?
http://www.airsoftplayers.com/accuracy.asp
http://www.airsoftplayers.com/accuracy2.asp