ToonBoom studio is cheaper (around $350 ish last I checked). Both are capable of amazing stuff but there are pros and cons with each. If you're going primarily for browser-based web delivery, you should use Flash IMO, because it's built especially for the web. ToonBoom Studio is capable of .swf export but there's no actionscript capability and not every cool effect can export successfully to .swf
If you intend to create movies for broadcast, DVD or even web video, then you can't go far wrong with ToonBoom Studio. In fact, if you want to make movies for the web in video formats (instead of .swf) then ToonBoom Studio might be the way to go.
Basically, with the exception of actionscript, ToonBoom can do most things Flash can do, plus a bit more. Likewise Flash can do certain things better than ToonBoom. If I was just starting out, I'd go with Flash because the Newgrounds audience is the best exposure for a beginner (worked for me). Then again, you could go with ToonBoom and find your audience on YouTube.
Both have free trials, so trying them out will help you decide, but then you may eventually be using both, like me.. depending on the task.
