I don't think you are. Mixing an 8 and a 4 ohm cab is inadvisable, which leaves the option of 2 x 8 ohm cabs (assuming a 4ohm amp) or one 4 ohm cab. 2 8ohm cabs will be louder and sound better but are more hassle to haul. One 4 ohm cab is easier to haul but you won't be able to push it as much, mainly due to mechanical limitations. There is also the option of using one 8ohm cab depending on how loud you need to get. Using one 8ohm cab will get you approx 350w max from a 500w amp. In practice this is often more than sufficient!
I'll always opt for one 4ohm (or 8ohm) cab because I'm a lazy bugger and in a relatively quiet band. I don't need to get loud. Getting loud is overrated anyway, imho.
Currently I'm using a 350w 2x10 combo which sounds great, gets quite loud enough and weighs 16kg for the win.