Take a look at this page: http://www.accuratereloading.com/22rf.html
It has group size averages from several different rifles and many different types of ammo. One thing I noticed was that the CCI Blazers (about $1 for 50) had groups that were among the smallest.
A lot of people that post to this board shoot the 550 round federal bulk packs (which are high velocity rounds, by the way). There certainly isn't anything wrong with shooting the cheep stuff. And as you can see from that list I refered you to, some of the cheeper stuff can be just as accurate as the more expensive stuff. I believe the more expensive ammo usually has less chance of duds, which can help out in a match.
I remember one story I read somewhere n this board which I will paraphrase. This guy was shooting a matches with cheep, bulk packaged ammo. And was doing quite well. That didn't keep the other competitors from making fun of his inexpensive ammo. So he started loading his bulk packed ammo into empty boxes that he found in the range garbage can. People stopped bugging him when they thought he was shooting the expensive stuff.
As far as the difference between the different velocity types:
First you have *Standard Velocity*, which is also sometimes referred to as Subsonic. Most of the match ammo is standard velocity. Then you have *High Velocity*, which is supersonic when fired in a rifle. And finally you have Hypervelocity ammo, which shoots a few hundred feet/second faster than the high velocity.
The reason, as I understand it, that the match ammo is designed to be subsonic is as follows. When a supersonic bullet slows down to below the speed of sound (which happes at obout 50 yards down range or so when fired from a rifle), it starts to tumble, which effects the trajectory slightly. So if the bullet never travels faster than the speed of sound in the first place, it never has this problem.
The thing to keep in mind with handguns is that with the short barrels that we shoot with, even the high velocity rounds only reach about 1050 fps, which is slower than the speed of sound. So we don't have to worry much about that transitional tumbling.
One final thing to keep in mind (than I will give my poor fingers a rest). Page nine of the Mark II manual says, "The Ruger Mark II pistol is chambered only for the .22 caliber Long Rifle cartridge, standard velocity or high velocity, munufactured to U.S. industry standards." Then it says don't shoot anything else through it. So you may want to stay away from hyper velocity stuff.