Luggage.
You don't want to be repacking your shit while late a for a flight because you're a kg over the limit and it's gonna cost an extra $100 if you don't and mistakenly rip the zipper off your carry-on. You really, really don't.
You don't have to completely break the bank either, but if you value your sanity, I wouldn't spend too much less than $200 on a carry-on/check-in pair.