[FrankHudson]

I'm not a great recordist, there are others who will respond here who get better results that I do. To help them, elaborate on the guitar sounding bad when you record simultaneously? What sort of bad?

My impression is that if you are using a single mic or single array of a pair of mics, positioning them correctly in a room that also doesn't detract is the key thing. It's hard when you "engineer yourself," but move the mic(s) around to get see what changes/gets better. You'll have to wear headphones while you do this. As far as I know and have experienced, this is extremely awkward to do as you self-engineer, but it's important.

The advantage of recording the parts separately is that you can fix mistakes with overdubs or editing easier, and if you want to apply effects you aren't trying to compromise between the vocal and the guitar. If you don't need to fix mistakes or apply effects noticeably, then don't worry about bleed, worry about what it sounds like before you hit record.

If you do need to fix mistakes, or to maximize the quality of the performances instrumentally and vocally, or for other reasons you want the vocal and guitar to be separate with no appreciable bleed, then you have to work out a way to track them separately. If you are a regular player with even rhythm and worked out song structures you can record the guitar and then on a second pass record the vocal, and Done!

If you're like me (sloppy player, irregular in his song structures) you may want to record the guitar and vocal with bleed as a scratch track. Then overdub a "keeper" guitar track using your original performance to guide you, and then finally a vocal record pass with the newly recorded keeper guitar track (sometimes I'll even keep the original vocal low down in the headphone monitor mix to help guide me with my irregular structures). If your DAW offers it, you might want to use a click track, or watch the bar lines on the DAW's screen to help regularize your playing. Some feel this detracts from the looseness desirable to some styles of playing, but these are tactics that can help some players in some contexts.