well for starters lets look at your charger, USB.... that's 5volts out... car halogen are 12volt, so either use step up (not good idea) or stick with LED the reason I don't think using a step up is a good idea is the possibility of blowing a USB hub. as this sounds like your first project try to stay low volts
now as for LED vs Halogen, the reason halogens aren't used much is they have less life span, and drain more power (watts in this case as they use both volts and amps more).
so LED is what I would use. as for using those batteries, if you are intending on using Lipo, then ensure you have a brilliant charging circuit or else they tend to get smokey, then burn, then house burns..... then fire department come... you get the idea
now the only thing you really need to think about is how bright you want it, look for "lumens" or "candlepower" and get one that is ridiculously big for ultra bright, blinding as the sun from the same distance light
good luck
