^^^^^^^^^^^ very good advice right there.
I own both - use both... doesn't mean I'm any good at either...
They are different animals. So the answer "depends".
Depends on what you intend to do the most - and depends on the size of your wallet.
If you can only afford ONE - a better/bigger MIG would be my choice. It will weld everything we do in the hobby... INCLUDING aluminum (with a spool gun). I have stainless gas (tri mix) and Stainless wire... so it will weld the "stuff" we use in the hobby.
I will ALWAYS choose to TIG over MIG given the choice... it's just more fun - makes less cleanup (grinding/finishing)... controls the heat better... looks better unfinished... but I'd never give up my MIG machine. They're just two completely different animals the only thing they have in common is that they're both "welders".
Having said the above -- a good welder (meaning the operator) can make beauty with any machine / process. Like a golf shot. It ain't the club, it's the guy swinging it (I suck at this as well).
If you can Oxy weld - you'll pick up TIG in a heartbeat. Very very similar eye hand coordination. Pushing a puddle - dipping the fill rod... controlling the heat.