I haven't pulled the trigger on this yet, but I've been doing the research for some time now and I'm pretty sure I'm going to go with a TIG set-up, when the time comes. That being said, a lot of ppl do just fine with MIG set-ups; you're just not as likely to get as clean of a bead, and it'd be more of a problem if you're working with very thin-walled tubes (like, on a high end road or mountain bike) than it would with the dookie-fat mild steel tubes most folks on RRB are working with....
HTH. If you search, I know this has come up a few times, Luke the Joker has used a variety of set-ups, and he gave pros/cons for each as far as the home-builder perspective goes. I think he said, with all factors considered, he preferred the MIG set up.
(As far as Harbor Frieght $100 welders--- I have no direct experience, but those rigs are often maligned on mtbr.com, and I generally do believe you get what you pay for....)