For the record, in the studio when recording close mic'd instruments (within around 1 to 3 feet) the difference between omi and cardioid can be quite minimal, particularly if the acoustic space is well controlled. The most noticeable thing will probably be the omnis more even bass-response as distance changes, while the cardioid will exhibit a greater proximity effect.
Nothing beats wearing a set of highly isolating headphones and moving the mic back/forward, left right, at various angles - while the player does their thing to find the sweet-spot. Indeed the sweet spot will depend entirely on taste...btw, about the guitar sounding too bright, do you have any objections to making EQ changes? I am always happy to cut frequencies...any chance of posting up a short example?
digifish