Lets say Capcom was going to make one or the other, and asked the fans to decide which they would prefer.

which would you pick to be made?

Now the basic is, its going to be a JRPG, turn style based, and have all the basics of an RPG (enhancement items (defense, attack, and other stat increasers), healing items, other status effecting items, ect...you know the basic bag of tricks twisted to match which ever was made/

you would know nothing else, like who the focus is, what the story would be, or any intimate details of the production. Other than the usual, will have side quests along with the main quest, monsters(or equivalent enemies) ect...

only that which was stated right here.

so,

which would it be?

SF RPG in my opinion.

Personally I would go for a Darkstalkers RPG, even though weapon wise guys like Ryu could just be said to have "gloves" like the fist fighters in Disgaea. At the end I can see the Darkstalkers fitting into an RPG better with a more fantastic (as in fantasy) story line in Makai, with a more diverse cast of enemies to fight (monsters and machines, ect..).

In the end what it comes down to really between these two, would also fall into the two main catagories of RPG games.

the Sci-fi RPG(Street Fighter) vs the Fantasy RPG(Darkstalkers).

Street Fighter would likely be a sci-fi RPG with some fantasy elements (like special attacks), using real world locations, machine enemies, androids, robots, soldiers with weapons, basic fighters, ninjas, ect... focusing on a plot simular to the games (Bison, or other secret group trying to combine the fighters powers with science, or simular plot line.

which does sound fun, but if If I had to choose, I'd go with the type I like better, which would be the more colorful cast of Darkstalkers and the fantasy RPG type of world, which would be magic, some over the top enemy out to take over Makai and/or Earth *typical story for demon characters*, a variety of classical, iconic, and original monster enemies, and some machines.

you know I would buy either one, but If only one was to be made and it came to a vote I'd vote for Darkstalkers. 

How about a Street Fighter vs. Darkstalkers 2D fighting game?

Or a Namco x Capcom Tales-style game? It has the gameplay of a Tales game, but with the SF characters, you punch in their combinations. Characters like Mega Man, Ryu, Morrigan, Dante, Viewtiful Joe, Amaterasu, Jill Valentine, and Phoenix Wright join up with Cless, Kratos, Pac-Man, and other Namco heroes against forces such as M.Bison, Vergil, Dahlia, Dr. Wily, Wesker, Dhaos, and other Namco and Capcom villains.

Truth be told, my knowledge of Namco games outside of the Tales seires is quite limited. However, if there were Eternal Sonata characters involved, I would be really happy.