Try to locate a pair of sm81's or Oktava012's. s
i'd nix the omnis until you get a pair of cardioids in case your environment is not ideal. i'd also look into small diaphragms to start due to their increased sensitivity...
I'd go for the 012s if I were trying to buy good cardioids and had a limited budget so I could only buy one pair. The SM81s are good on some really meaty-sounding guitars, but you can get the same effect with a simple low cut on the 012. The 012, however, tends to be a better general-purpose mic, sounding better on a broader range of instruments and other sound sources.
I'm not sure I'd agree about the omnis vs. cardioids. It's a little counterintuitive, but omnis can sometimes pick up less of the room with instruments than cardioids, depending on placement.
Cardioid mics change their response characteristics when placed close to a sound source. This means that you often have to back off from the source to avoid a boomy sound. Omni mics don't exhibit proximity effect, so they can be placed right at the sound source.
Depending on the mic, cardioid mics may reject as little as 15 dB for sounds coming directly from the back and far less for sounds from anywhere else. It's important to keep that number in mind. I'll refer to this as the signal to noise ratio delta because that is the improvement in signal (direct sound) to noise (reflected sound) due to using a cardioid mic.
Omnidirectional sound emissions (like any other omni emission) falls off with the square of the distance. That means that distance from the source makes an n^2 increase in the sound level. If removing the proximity effect allows you to place the omni mic at half the distance, your sound level increases by a factor of 4, or a 12 dB signal to noise ratio delta relative to that same omni mic at the original distance. A factor of about 2.8 (sqrt(8)) gives you an 18 dB SNR delta---more than the delta provided by many cardioid mics even for signals directly from the back.
Whether that applies to guitar recording depends on how far you have to back the thing off to keep the guitar from sounding boomy, which depends in part on the guitar and the sound you're trying to get from it.
Either way, at the distances most people mic a guitar, the room shouldn't play a very big part even with an omni mic unless you're in a really tiny room or you don't have carpet. If the distance to the mic is three inches and the distance to the nearest wall is even four feet, you have about a eight foot round trip distance for the reflection and it is down by 30 dB relative to the direct sound, which is right at the limit of being able to hear it.
(Please correct my math. This is back-of-the-napkin stuff, so it is subject to being seriously wrong.

)