I generally don't like the EF-S mount since you can't use them on the older film SLRs.
Don't think you can use them on the 5D either. If you intend to upgrade to a 5D in future your EF-S lens will be useless.
Otherwise which lens you get depends on what telephoto range you want.
I had the same problem deciding between a EF 75-300mm f/4-5.6 III USM and the EF-S 55-250mm f/4-5.6 IS. In the end I went for the EF-S 55-250mm f/4-5.6 IS because of its light weight & image stabilization. To me IS is important caus I tend to have shakey hands when I've been shooting for extended periods. If you intend to upgrade to a FF camera then getting an EF lense might be a better choice.
Why not you also consider EF 70-300mm F4-5.6 IS USM or EF 75-300 F4-5.6 III USM?
Because I think these 2 lenses will better complement your current 17-85. The focal lengths will overlap a little, but not as much as 55-250 to amount to a waste. Plus with USM, like your 17-85, you'll get fast and quiet focusing.
I've tried the 55-250, it is a good and decent lens, but sharpness is poorer than 17-85. And like others have said, it will not work on full frame bodies.
I've not used the 90-300mm before, so I can't comment, but if you buy that lens, you will be losing 5mm in your total range. And without the IS, you may be disadvantaged. However that actually depends on you (e.g. whether you have shaky hands). I'm fine without IS.
I'm a satisfied owner of a 55-250IS and can say its a pretty sharp lens for the money. Image Stabilizer is crucial for dimmer conditions. The 75-300 has a good range but only useable up to 1/300 or slightly slower (1/200 ?) if you have steadier hands. I feel IS is quite important for such a long focal length.
EF-S 55-250mm, a modern telezoom lens specially designed for 1.6x body starters. Coupled with IS at such an affordable price... good image quality vs the price... lightweight and reasonably fast AF... for me it's a pretty straight forward selection
BTW, the USM in EF 70-300mm/75-300mm is not those ring type, so, this option is void to me.
The useable shutter speed of a lens really depends on how steady your hands are. Holding technique also come into play. I took pretty good shots before at 1/40s F5.6 with the 75-300mm non-IS.
The 55-250 is overall a good lens. It is made not with superb build or superb quality in mind, but rather affordability and usefulness. The IS can be very helpful. Unfortunately it only works with cropped sensors.
Hmm I see, then what type is it? Personally, I will value USM more than IS. Faster focusing allows me to do more kinds of photography. Can do sports, birding etc. I tried capturing sprinters on lenses without USM, the lens cannot make it - too slow. Haha:bsmilie: