By all means get a flash, but as others have said you shouldn't expect to be able to use it in public much, so it won't really solve your specific problems - museums and popular churches, for instance, won't allow flash photography at all (nor tripods, for that matter); they irritate other people; and the results seldom look good anyway (there likely won't be a suitable surface to bounce it off) except in very controlled situations such as you don't describe.
I've not tried the Sigma, but I do own the Canon 28mm f/1.8 which tends to get similar reviews. I've not tried it on my Rebel, but the photos I took with it inside Notre Dame Cathedral (which is pretty dark) on my 5DII at f/1.8 are horribly soft around the edges (at least those will be missing on a Rebel) and not that impressive elsewhere; I got much better results with the 24-105 L and 70-200 f/4 IS L at f/4 (their IS helps, of course). The 85mm f/1.8 produced far superior photos than the 28mm, but based on what you've said that may not be wide enough; if it is, I would certainly recommend that. I like the 20mm f/2.8, but that's not brighter enough than what you already have to make a significant difference.
As for the Tamron 17-50 f/2.8, I owned one of these when I owned a Pentax K-5, and when it focused properly (a chronic problem with Tamrons, evidently) the results were excellent - but the K-5 has built-in IS, which helps in low light; I've not tried using it without IS. Reviews indicated that the IS version of the Tamron (they call it VR) isn't as good optically as the non-IS version. I suspect that the Canon 17-55 f/2.8 would be better than either - if nothing else, it will focus better and has IS - but like the Tamron, at the wide end the difference in aperture compared to what you already have will have little practical effect.
You complain about the quality of low light photos taken from rides. Assuming the ride was moving, if the light was low enough there might not be any lens fast enough to cope with that!
One last thing - as someone else mentioned, if you're not doing so, make sure you shoot RAW (or RAW + JPEG); conversion software will likely do a better job of noise reduction than the camera's built-in software (I like DxO for this). My Rebel t3i isn't as good in low light as my 5Dii, but I took some photos inside the Madeleine (a really dark church) with the 70-200 f/4 IS L which, with the help of DxO, look remarkably good, all things considered.