It depends what type of sound you are looking for. If you are looking for a bright sound of a guitar that is picked, you'll probably want a small diaphragm condenser. If you want a big, bold, full sound of a strummed guitar, then use a large diaphragm condenser.
I typically have good results with an Audix ADX-51 (Small diaphragm condenser) if I want a delicate, crisp sound. I also use an Audix CX-111 (Large diaphragm condenser) for that big. bold, full sound.
If you stick to well known mics from well known makers, you likely won't have any complaints about your choice.
There are mics that will perform very well in every price range.
In a reasonable price range, I would recommend that you check out Shure's KSM series, Audio Technica 20, 30, or 40 series, Rode NT-1a, or NT-5, the Audix mics I listed above, AKG C451, and many, many more. Heck...even the classic Shure SM57 will give acceptable results.
There are far too many good mics that would work well on guitars. It will be up to you and your price range as to what will work the best.
If you plan on using the same mic with horns, you'll want to find one that has a pad switch on it to keep the mic from overloading with the higher volume.
Jeff Foster
Technical Director
Northside Baptist Church
Carrollton Texas