I asked this same TIG/MIG question to the owner of the fab shop that is fixing the cracks in my boat. He has 20 years of experience in aluminum fabrication and will only use a TIG when working on boats. He feels it gives him more control and provides a cleaner weld.
As far as filling in holes, he prefers to cut our a nice even square around the bad area and weld in a patch piece.
With all that said, he told me a good MIG with the right gas, wire, and spool gun set-up will work fine too.
I don't have either, and I don't know how to weld. For me, its a heck of a lot cheaper, faster, and better to just let the fab shop do my welding. Its only costing me $100 to have four cracked ribs and a 12 inch seem split welded up.