50 cal and new players/kids
I recently took my 7 year old twins out with some some friends for a paintball party. We used 50 cal spyders and they had a blast. Even with 50 cal we had a couple of cryers after getting hit but nothing like would have expected with 68. We did one game of everyone vs Dad and when I finally got shot I barely felt it even at close range ( I was running around shooting at bunkers giving them something to shoot at). We were indoor so the ranges were pretty close most of the time.
For new players and kids I think 50 cal works great. They dont get the difference in the range and the lower impact and less hassle about reloading works perfect for that type of group. I see it as a great rental/private option. Once you are ready to play with the big boys, get a 68 cal marker and go play at 12+bps but for new players is a great introduction.