I'm going to be the odd ball here...personally I'm a very big fan of Tamron. Up till recently my main lens has been a Tamron 70-300mm f/4.0 - 5.6 LD (no IS) which I got on Ebay for $65 used and I have gotten some amazing images with it. If you look thru my older postings, you'll see a great many images shot with that lens and as they say, "the proof is in the pudding". Most recently I picked up the new Tamron 70-200mm f/2.8 ($700 on Amazon) and again it has been a truly exceptional lens to say the least. I also frequently use my Tamron 28-80 which I've also gotten some really great results with.
My suggestion would be that if you can live without the IS, I'd go for the Tamron 70-300mm LD. Even new they can be had for well under $200. I have two of them now (the first one broke on a sidewalk when the strap on my camera bag broke) and both have been quite exceptional for the money. If you really need the IS however, I would seriously look at the Tamron 28-300mm f/3.5-6.3 XR Di VC. I haven't used one personally (yet) but they got some fantastic reviews when they first came out. While at f/6.3 on the long end it is a little slower than the Canon lens you mentioned, it's actually faster on the wide end plus, it's wider on the wide end (over 40mm wider), it has one of the longest zoom ranges available in any price range and it has a 3 axis image stabilization system as apposed to Canon and everyone else's standard 2 axis system. For $600 new, it's a very hard lens to beat!
Honestly, if you are on a budget Tamron really is quite hard to beat. Yes, they tend to focus a little slowly (the lower end Tamron's will focus on par with your nifty fifty) and at least my two lower end Tam's are a bit noisy...although no where near as noisy as my Sigma 18-55 (and my new 70-200mm f/2.8 is fast and very quiet). When it comes to image quality and sharpness, all of my Tamron's have been on par with lenses costing a great deal more...including many high end Canon's. Please feel free to look thru the images that I've posted here on POTN and if you would like I can even send you more...don't let people BS you into spending more money than you have to to get a sharp lens
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Now as far as the wider vs. longer issue that dave mentioned above, that should be determined by what you plan to shoot. As he stated, if you're looking to shoot large groups indoors or even if you're looking to capture sweeping landscapes and sunsets, you may be better off going with something wider than the nifty fifty (also a fine lens BTW). On the other hand, if you know that you're going to need the longer reach...say shooting critters at the zoo (or in the wild) or something, then the 70-300mm is a very good range to start with. Just my opinion here, but focal length should always be determined by need.
I know I'm starting to sound like a sales rep here (and I'm honestly not) but if you have any questions about Tamron, please feel free to either PM me here at POTN or drop me an email. Alrighty, I hope this helps!
Peace,
Jim
"It is horrifying that we have to fight our own government to save the environment. " - Ansel Adams
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