Over the past years, I have used all 3 of the major tax programs, but I always buy the boxed version. Personally, I like taxact and hr block the best; they seem less cumbersome to me, and both have always been cheaper. I think taxact is actually the cheapest.
I like the boxed version because I also do tax returns for other family members, and print them out for them prior to filing online. Not sure if its still true, but it seems to me that with turbo tax (boxed version I'm talking about here) there's a limited number of returns you can prepare with it.
I always get the boxed version, because I like to print out the various reports, schedules, etc.; set them aside; then go back and review at a later time for accuracy. It's pretty easy to transpose numbers, and somehow I feel when you just plomp yourself down at the computer to file online, it would be to easy (for me, at least) to make a stupid mistake.