Lens Buying help...

So many knowledgeable people here with so much experience with so much equipment. I want to take advantage of it and get some help with a new lens purchase.
I am looking at the Sigma 28-70 2.8 EX DG, Sigma 24-135 2.8-4.5, and the Canon 28-135 3.5-4.5 IS USM.
Here's my situation. I shoot people and landscapes mostly. I like the reach of the 28-135, and I particularly like the IS feature on the 28-135 Canon. My priorities (in order) are: sharpness, range (focal length, hence the reason for favoring the 135 length) and aperture. I already have a long zoom and have the longer focal lengths covered, but I like having a lens with the range of the 28-135. I have used the Canon IS 28-135 and I think it is a very good lens with good sharpness. The IS feature works well. The Sigma 24-135 has the same range but no IS nor is it EX. It does have some Aspherical and SLD lens elements though. Is it reasonable to expect that the Canon IS will be generally sharper than that lens? And the big question, does the the Sigma 28-70 EX DG have a level of sharpness that trumps them both? If the consensus is that the Sigma 28-70 EX DG has sharpness considerably better than the Canon IS 28-135 and the Sigma 24-135 I would give up the range for the sharpness. If it's more or less marginal, then I wouldn't.
I know that lens buying is 75% personal needs and preference, but I'm looking for help with the other 25%. Your experience and opinions are appreciated.

I owned the Canon and it's Ok. I actually sold it within a week because I knew straight away it didn't have the quality I want. I haven't kept a single shot from it. Maybe that's your 75% personal thing! I then bought the Tamron 28-75 which is certainly a better lens optically but lacked enough for me on the wide end. I'd have kept it otherwise I think. I next bought the Sigma 24-70 ... I still have it and am happy to recommend it. I trust it will do the job and it usually does.

Anyone have any experience with the Tamron 24-135 3.5-5.6? The reviews over on FM rate it pretty high amongst users. It might not be as popular as the faster lenses, but that isn't so much of an issue for me. I need sharpness and clarity at a less-than-L price.