I remember seeing a graph someone made for this.
It compared the speed vs the distance between .2g and .25g pellets.
I remember prior to 90 ft, a .2g pellet is travelling faster, after 90 ft, the .25g pellet is travelling faster. The .25g pellet travelled furthur I think due to it's sustained fps after 90 ft, but I could have remembered wrong.
So if you're playing cqb and other smaller fields where 90 is the maximum distance you'll want to shoot, a .2g pellet will actually get there faster (well...at 90 ft they will get there together).
As for the pain factor, I haven't seen any difference in actual play unless you shoot someone at something less than 30 ft. Even then, it's not a terrible difference.
***Ah, I found it:
http://www.arniesairsoft.co.uk/?filn..._dist_time.htm
If you read it, you see that at 50 ft, the two weights have the same velocity, but the .2 has travelled furthur. At 90 ft they travelled the same distance.
Now if you translate that into energy, like I said, anything over 30 ft, you're pain factor is almost the same.