9V batteries can only source a very limited amount of current, usually around 150 mA; a single AA battery can provide much more current, but at the expense of low voltage and weight.
I would like to install a lithium-ion pack into a helmet in the future; Li-Ion packs, while more expensive, can provide a LOT more current than a 9V in a comparable space. For example, most cell phone batteries are 3.7V ~1000mAh; two of them in series can probably power a correctly sized fan for a while. Downside of rechargeable batteries is the special charger needed (especially for lithium-ion), and a 9V is just more convenient.
I'd stick with the 9V in this case. Remember that your two fans are each drawing current, and I assume you have them in parallel for maximum power and airflow circulation; that draws a ton of power from a standard 9V battery. You might want to look into lower-current fans; motors use more current than almost any other standard device, and that's the big problem here.
