Nikon 5100 50mm f1.8 OR Canon T3i with a 50mm f1.8?

I expect clear sharp images with better ISO capability… And I'm into stills than video most of the time.
Which combination will I go for?

It doesn't matter - either is perfectly capable.

Where you're falling down is deciding to equip your camera only with a fast, prime telephoto. The 50mm f1.8, while an excellent lens when used properly, is a supplement to the 18-55mm standard zoom, not a replacement for it. That's why I carry an old 50mm f1.8 Pentacon and an M42 adapter (not suitable for the Nikon), and not a three-figure prime lens I'd use for one shot in ten, if I'm lucky.

Nice camera

Since Nikon have 10% larger sensors than Canon, you should expect a bit better low light capability with the D5100 - which can be confirmed by DxOMark sensor ratings.

http://www.dxomark.com/...2%29/Canon

Everyone wants to tell you that all DSLRs are equal, but Canon sensors are smaller than Nikon, Sony, or Pentax sensor. While the difference may not be significant, why pay for something that will not provide as much performance?

You're going to get lots of different answers most based on personal preference.

I happen to be a Canon guy and I own a 50mm f/1.8 and I find it gives me exactly what I would hope for, well exposed, sharp images in lower light situations… But I suppose the Nikon will do the same.

Either or… I'd go for the Nikon myself.

Doesn't make a bit of difference, DxOmark or not. Measurebators aside, you won't be able to tell the difference between the two in print or on a monitor. There's the issue of the crippled D5100 not being able to autofocus without the specific AF-S version of the lens, which costs more than the same D model without AF-S.

Sony, Pentax, and Nikon all use the same sensors… Made by Sony.

Do not expect from the camera. Clear and sharp is your responsibility. Any camera is capable of clear and sharp pictures. It will all depend on your skills.