To divert from the thread title slightly... :-)

I found open-G easier to learn. Open-D has 4 of the 6 strings detuned, so you basically need to relearn everything. Open-G only detunes 3, and the 3 left happen to be DGB in the middle which are your main chording strings. So most of your chords are reasonably intact in open-G (or enough to fake it anyway) and you can reuse a lot of your existing habits. Also the bottom D/G are the same interval as E/A in standard, so bassline stuff is a bit more familiar too.

Plenty of blues in open-G. Also the classic "Romeo and Juliet", for which you can doubtless find tab on the web.

Graham.

PS. No capo?! How do you sing to the guitar? I know it's possible to play in any key, but don't you sometimes find you want the particular texture that you can only get from a certain chord?