Lens Swapping Suggestions

I am wondering whether I should swap out some lenses. I currently own the Canon 28-135mm IS lens. While this has been a good lens, I really wish that there was more at the wide end to go with. I am currently wanting to get into more street photography type work, so it just does not adequately cut it. I am thinking along the lines of getting a Tamron 17-50 f/2.8.

At the same time, the 28-135 makes a pretty good walk around lens for places like the zoo or the nature center. I think that I would miss the IS and the versatility of a large zoom range of I make the trade. Unfortunately, I do not have any extra money to throw at other lenses, so I am unable to keep the 28-135 IS if I want to pick up the Tamron. I am thinking that in the future if I make the trade, I will end up going with either a Sigma 18-200 OS or a Tamron 18-270 VC.

Any suggestions based on personal experience would be appreciated.

Or maybe the 18-55 IS and the 55-250 IS would be the better choice? (both canon versions.

i bought a Tamron 18-270 VC after trying it in the store. It is well made and does not scream "expensive lens" like the white L lenses. I used it as a one-lens-fits-all solution to photographing my daughters in a marching band. Tough to stop their motion (they are constantly moving) but when they stand still the VC works better than any IS I have tried. I had the 28-135 IS and sold it, just not sharp enough.

So you think that the 18-270 Tamron is a sharper lens than the 28-135mm IS?