I think it's almost certain that the bundle will be cheaper than buying them seperate, so I will get them together. Is the Zapper still coming with Ghost Squad as well? With Capcom including it with UC, that might damage the sales of Ghost Squad I think. I for one was only going to get Ghost Squad, if it was a decent game, and I saved a good amount on the Zapper. But now I might not get it, but then again I might so I can have two Zappers for multiplayer (depending on the price of everything, I can't be bothered giving figures for an example!).
I really like the idea of UC being on-rails. It's by no means a replacement for RE:5 and it is not the next chapter in the story. As JeremyGTS said, the game is showing the whole story of the earlier Resi Evils, so it makes sense being on-rails as it isn't one in-depth chapter of the story, its a few put together. I don't think there would be much point in just re-releasing the earlier Resi Evils in one game. It's more of a 'fun' game to tell the whole story, with some extras I believe.
Plus, on-rails is fun in my opinion, you can just pick it up and play for about 20 minutes and then walk away. Whereas with a 'proper' Resi Evil games, it's hard not to play for a lengthy amount of time. And with the Zapper it should be awesome!