You mentioned extension caps, are you using a ProVari or some VV mod? 2.5 Ohm at 4.5V is a reasonable load, 8.1 Watts pulling 1.8 amps from the battery. You shouldn't require high drain batteries for that, but I'd still go with them. My setup is comparable and I haven't used anything other than IMRs for over a year now.
IMRs have a lower energy density meaning they hold a smaller charge than regular Li-ion cells (measured in mahs or miliamps per hour), but the inherent chemistry used is a lot safer -- so much so they don't require protection chips, making them smaller than regular protected Li-ion. They're also "high drain" (can sustain a much higher continuous power draw) meaning they'll perform just as well if you decide to use a lower resistance setup or just punch up the voltage.
In my experience, IMRs also "spend more time" in the higher part of the voltage curve -- they seem to take longer to go from 4.2V (fully charged) to 3.7V (nominal voltage) than 3.7V to 3.2V (no charge) compared to regular Li-ion. And unless you're a really heavy vaper, a 2000mah or 2200mah (like the latest panasonic cells) IMR should last you all day, or close enough as to not make a difference. Taking a fresh spare with you is easy in any case and you should be doing it regardless of what battery you go with.
All in all, as long as you get a battery from a trusted vendor (or trusted brand), especially if you're using a decent mod (like a ProVari) it shouldn't make much of a difference. But keep in mind that as with most things in life, you do get what you pay for with batteries, so don't be too cheap. A more expensive battery may perform better and last longer, if properly taken care of, which might well offset the higher initial cost.
You don't have to buy AW, though most people here will tell you that you can't go wrong with them (I use them too). Panasonic cells are fine too (in fact, I think AWs are simply rebranded Panasonic cells), Trustfire should be a safe brand, etc..
Above all, unless you buy IMR (I means they're Lithium-ion based, M means they're the LiMn type or manganese, R is for rechargeable) or LiFePo (Lithium Ferrous Polymer, or something like that), both of which are considered "safe chemistry" batteries and thus are inherently "protected" not requiring an added protection chip, don't buy regular unprotected Li-ion batteries especially for use with a mod (that tend to stress the battery more).
