You say you have both frames now, but not from the sound of it a touring frame in 700C. While MTB frames make good tourers, not all MTB frames do. If both frames where ideal geometry for touring, I would go for MTB if any offroad or roughroad, and I would go touring for smooth sailing. From that I would then decide on the tires, because the issues are basically the same rugged tires and out of the way places, favour 26" and greater ease of sourcing. More upmarket roads and terrain are probably going to favour getting 700c, though touring grade 700c is difficult to get everywhere.

There are a lot of good rims out there that are perfect for touring. I used alex rims, and they didn't budge, under my considerable weight. I think abuse is far more important than touring. For me I try to keep it smooth on a long tour. I think one covers more ground that way. I think well build rims and huge loads are fine as long as you keep it smooth. If you can't keep it smooth, I would be looking for fat tires like 1.9 slicks, or suspension.