I'd go for the 5lb extinguisher. If you only cared about fires at start-up then a big 10lb one is fine, but when you need to grab that puppy and run like the clappers to get to an airplane that's burning some distance away from you, you'll wish you had the smaller & lighter one !!

5lb is also enough to put out the majority of fires I've seen. (not counting massive grass fires etc - but for those you need your water extinguisher anyway, not CO2)

CO2 is probably a better bet than Halon - I have both ; CO2 is easy to get a refill for, but Halon is becoming ever harder to get.

Gordon