The Vaquero is basically a blackhawk with a different look. The old Vaqueros will handle almost anything you feed it.
The Blackhawk may be the better choice if you want the adjustable sights. Also the Black hawk can be bought with another cylinder to shoot 9mm or 38/357's.
If you are looking at the Vaquero I assume you like the look of it. I own one, I bought it because it was cool (Birdshead 3 5/8 barrel in 45 colt) Its fun to shoot and hits where I aim it.
I would get a double action if my primary concern was self defense.
There is a good reason to own both, if you have decided on that I would get the Vaquero first then the GP-100 later. I would choose that way only because I like the coolness of the vaquero and I can use it for home defense. Also you can run loads through it for hunting. The GP-100 second for a nice carry gun that is solid.
Your needs may be different than mine.
You can use the single action as a conceal carry, but you must practice cocking it, its a skill. Go to youtube and type fastest cowboy shooter and watch them. But it takes practice.
What it comes down to is what do you need it for?
For plain old fun and solid, the Blackhawk or Vaquero, but thats my opinion. (actually the Vaquero just because I think its cool)
(I own two double action 357's one is a Dan Wesson, a Dan Wesson 44, two single action 45's Uberti and Vaquero, a 357 Blackhawk, two 1911's, SR9, 380, and a Ruger MKII )