Depends on how many reliable attenders you have. Even with reliable attenders, things happen that conflict with tourneys - funerals, weddings, vacations.
If you have 3, then stick with 3 (duh). If 5, stick with 3 for now and maybe try for 5, knowing that you'll likely need a replacement at the last minute (because crap happens).
If you have 5, why not play both?
You asked about which are more prevalent. I was going to ask the same of you. Around here, there's some alternation of 3 and 5, so can get some of both. A couple fields cater more to 5, others more to 3. You'll want to find out more in your area.
Some fields are delaying their tourney schedules for this year. I imagine it's mostly uncertainty about the economy. I know my favorite field cut several tourneys in 2011 because of "lack of interest." I'm hoping 2012 is better.