Finding the right one is hit and miss. I have bounced back and forth between Hollister and Convatec over the years. I like the drain clips Hollister uses better than Convatec as they are less likely to open unexpectedly. Hollister worked well, but a couple of years ago I had problems with them sticking to my skin and switched to Convatec.
I found the two piece system better as I get quite a bit of gas, and airing off the bag at the flange is easier than through the drain of the pouch one has to do with one piece systems.
There are some with vents, but I find they fail after one day due to my watery output.
Prices vary a lot between retailers, I found that some online colostomy sites have better pricing and free shipping.