Hello Frank.
If you've already made up your mind, far be it for me to cast a shadow on what is a beautifully compact and optically very fine lens. With my 10D, I got great results at all apertures and ISOs.
However, if you are shooting environmental portraits and candids, you may want to consider that the focus motor is not only very loud -- it turned heads on a loud commuter train moving at 100 kph while I shot unobtrusively through the window -- but also harsh sounding.
If you are planning to engage your subjects in a visual dialogue of potraiture, prepare for unexpected frowns and nervous smiles every time your lens sings its song. And forget being a fly on the wall. You will be a hornet's nest in the room. It is much more obtrusive than the 50/1.8II, which isn't the quietest lens to begin with.
Then there is the issue of build quality. My focus ring no longer moves the lens elements, neither at M nor AF setting. It's broken. I may have it repaired at some point, but I have read elsewhere that the cost may be equivalent to that of a good used sample. I also hesitate because of the motor noise issue. I simply don't miss it for photographing people even though that was my intended use.
Also, if you like to attach a lens hood easily with minimal micro-motor skill, and with a positive feel inspiring confidence that it will not fall off, you can forget about using the included hood which is temperamental and unstable. I used a small Nikon metal hood, but eschewed a filter, and it may have been the hood's weight on the non-rotating but extending front barrel which overtaxed the apparently delicate focusing system.
I also own the Sigma 28/1.8, which is a great lens as described by posters above. But I sometimes have focus issues on my 10D, a body which, although being notorious for its focus issues, nevertheless focuses my other Sigma and Canon lenses fairly accurately. However, you might be fine with your camera which uses the DIGIC IV processor as opposed to the 10D's DIGIC. That being said, if you have a chance to try the lens you want with your T2i in the store before buying, you might save yourself from possible issues mating the lens to a body that has no user controllable micro-focus adjustment. Buying from a reputable online dealer with a limited return policy may be another option.
Once I to upgrade to a body with DIGIC IV and MFA, I will dust off my own 28/1.8, and also give the gorgeous 30/1.4 a second look as a prospect. The mere mention of it above had me drooling. I know the price difference is huge, but it sounds like you have a specialized and enduring use for it. I would seriously consider selling a non-"L" kit zoom if you have such a lens in order to help defray the cost of a 30/1.4.
I understand your desire for the most bokeh for the buck, and I plead ignorance of the physics involved, but it is my impression, flawed or otherwise, that real-life bokeh as affected by the subjective bokeh quality of a particular lens mated with a particular camera sensor with all of its filters and particular pixel density and pitch may be far more relevant than what a comparison of depth-of-field tables can reveal.
I don't have any experience with the 35/2.8, but my FD mount Tokinas were well built, and their lens element coating turned out to be far more resistant to fungus than that of my Canon and Tamron FD mount lenses.
If I ever repair my loud and obnoxious 35/2, I will never again use a hood or filter on it, and will always switch it from AF to M before putting it in the bag or tucking it in at night. It is a fragile thing of beauty.
Just my two cents.
