i remember that we have a few herb/supplement knowledge people here.
Well I guess that would be me, but I have to admit probiotics were not my strongest area!
However, you get what you pay for pretty much. The cheap stuff isn't as good as the mid-range and more expensive stuff. Different strains go to and support different areas (stomach, small intestine, large intestine, etc.)
Are you taking it just for general gut health or to correct an imbablance? Usually for correcting an imbalance (say, you've been on antibiotics and your good flora has been wiped out) I'd go for the stronger and most likely more expensive stuff.
Some probiotics are meant to be kept cold. Some are stable at room temperature. I always felt that the ones that must be kept cold were better, though many would disagree with me.