depends on how much value your bankroll has.
if $70 is nothing to you and you can easily reload more money if you wanted, then play whatever stakes you want.
if you want to make the $70 last as long as possible and not risk losing it quickly, then you want to have more buy-ins per stake to reduce the risk of going broke.
with $70, if you play $5.50 mtts, you only only have 12 buy-ins. true, it's possible to win big and make something happen with 12 buy-ins but you also run the risk of going broke as a 12 buy-in downswing isn't uncommon. to reduce the chance of this, you'll want more buy-ins. so $1.1 mtts will give you 63 buy-ins (better) and .55 mtts will give you 127 buy-ins (even better). and if you want to go the very conservative route, you can even do .10 - .25 mtts, which will give you 280 - 700 buy-ins. this might seem like over kill, but it's a lot harder to bust your bankroll with 280+ buy-ins than it is with, say, 63 buy-ins. and players have busted with 63 buy-ins.