Not quite sure!

If you are already pretty experienced in anal why not get the progasm this unit is nice and works well. It is the largest but to me fells pretty good when i have a mini o i have yet to have a super o with any aneros but the progasm is my favorite for when i want a full feeling and the one i can just insert and let it auto pleasure me. The smaller units i tend to need to do a few small contractions to get them going. I say go for the largest then if you like it try the smaller ones.

My overall favorite is the Mgx its best for my anatomy it pleasures all the points i like. The progasm is sorta my quicky fix it seems to work fastest but its also over faster. For example with me i can get a mini o soon as 2-3 minutes after insertion but then i only have 1-3 mini o's then i am done there are usually no other sensations so i tend to take it out faster than the smaller ones. The mgx i can have a mini o but it takes a good hour or so for them to kick in but then there are more like 5-7 mini o's over a 2-3 hour period.