[keith.rogers]

I have probably only had the same make/model mic on my guitar and vocal once or twice, and that was when the only mics I had were two Beta 57as.

I would get the best mic you can afford that improves either your vocal track or guitar track (or both!), recognizing there will be a lot of bleed in both. This generally isn't a problem, though if you get into any plugins that do time shifting, like trying to add chorus to the guitar track, you'll find out quickly it probably isn't going to work very well.

Similarly, for stereo, if you mean two mics on your guitar, I wouldn't worry much. You will generally not be panning those wide, so differences in the mics can generally be managed in the mix very easily. I sometimes record with an AKG LDC and an AT SDC on the guitar. No big deal.

And, if you want to record something big, like a chorus or large ensemble, these two could be used there.

Digression - Where I get a little concerned about using different mics for a wide spread stereo mix is if it's a small source and the mics are very different.