I recently moved to Boulder and I'm looking to take advantage of the amazing trails here. I've started my research, but I know close to nothing about how to choose a bike. Since I'm just starting, I don't want to spend too much money.

These are three options I have so far. Any comments will be greatly apprecciated.

a) New Trek 4300. It's a woman's bike. It's being offered at $349.

b) Used (2 years) Iguana Giant. It's in pretty good condition, just scratched. It's definitely been used a lot. The guy went down to $375.

c) Used 2002 Surly Instigator. Also, in good condition: just the usual scratches: $500
>>Mavic 317 disk wheels with XT hubs
>>Avid Mechanical Disk brakes
>>XT rapidfire shifter pods and brakes
>>RaceFace Next carbon crank/Raceface BB/XT cassette
>>Marzocchi MX Comp W/eta fork
>>Cane Creek headset
>>Intense lock-on grips

I really want to get a bike already to hit the trails. Any recommendations outside these 3?
Thanks so much!