I pump to see where i would like to be for my third round. i have visual asthetic i want to get to. but the pump normlly last 45 minutes. i think it gives minor perm gains dependng on how long you do it.
I use a leluv pump and vacuum gauge. That way you know what pressure you are pumping to unlike the bathmate (I've heard max pressure is somewhere in the 12 range which is too high in my opinion). The leluv tubes are relatively cheap and offer multiple sizes for each individual.
As for temporary gains, I can typically get 1/2 to 3/4 Inch gains that last about an hour after 3 sets of 7 minutes at around 5 hg.
Just measure your diameter and size up from there. I really just did it by trial and error. Got a 2 Inch tube and it ended up being a bit too small (I would pack it after a sessions) so I upsized to a 2.25" tube which seemed a bit too loose. Ended up with a 2.125 tube which seems perfect. I tend to pack it half way by the end of a session. The tubes are cheap and I will likely need the 2.25 when I go back to Avanti for a second round.
You can purchase them on eBay and the shipping is relatively quick as well. The 3 tubes I have were about the same price as a bathmate (I own one as well) and I had a mitivac vacuum pump for changing brake fluid on my cars so I used that. You can just purchase the leluv vacuum pump and that will work just fine.