How much cheaper is the Aero? If its less than 30% the cost of the Boxer, than I'd consider getting the Aero. Here in the US I just sold an old Aero 12m in very good condition for around $250, I wouldn't pay much more than that. I loved these kites so I kept my old 10m Aero as a thrasher/lender kite, and in case my girlfriend decides to give kiting a try.
Your first kite is going to take a beating, and you are pretty unlikely to notice any of the new features until you get better, so you might consider an older kite like the Aero.
That said make sure its in good condition, and make sure you get an updated bar & lines setup
. Don't use the bar that came originally with the Aero, it will be outdated and not equipped with the safety features of a newer bar. http://www.bestkiteboarding.com
has good deals on bars and lines if you want one cheap (get the regular bar not the swivel one.)
Anyway, the old Aero is a great kite IMO. It relaunches easier than any newer kite I've flown, it has heaps of grunt (great for waterstarts), and its pretty damn stable for an older kite. Boxer is no doubt, two generations better, but the more you pay the madder your going to get when you thrash it. Maybe let the cheap kite taking the beating and get the nicer one later on.