Trinity;
You already got good deal of accurate information. Well I have not much ideas about smaller cousin, but personally own a 28-300 and let me share a few facts with you. The pros: Lens is the smallest in the industry for the rang it offers, and even is lightest, and reach to an unbelievable length at 300mm that you will be shocked as to where a these extra length was hidden...
The Con and by far important thing you want to pay attention is you get what you pay for. It is a good lens for when light is your friend and you have luxury of closing the diaphragm at least 2 stops. It is SOFT and I mean it. I went through a rough time when this was my main lens and only after paying for some L series start to see sharpness even in my dreams
. Well this is after becoming a lot poorer...
Lens is fine up to 200mm or so and after that you want to completely avoid the wide open F stop in hight focal lengths or a very soft picture is all you get. Remember that F is variable so when you get to hight levels of focal length the F is much smaller. I always thought this should make a very good combination to have XT, the lightest DSLR in addition to tamron's 28-300mm for a photojournalist on run who may want to take picture of critical zones when mobility is a mater of life or death such as Iraq or elsewhere, but to my surprise even the cheapest smallest news companies photographer prefer to use much heavier D1 D5 with something like 16-40L and 70-200L IS. May be they count on the mere weight of equipment as a potential projectile weapon if in need
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Of the topic but seriously consider the quality loss you get in view of money you save and also comfort of weight you carry.
If you need my advise. Go for at least 2 lens instead of one very wide range. A good and relatively cheap combination in my opinion is Canon EF 75-300 (Or same range Sigma or formerly called Quantarary) Non IS that here costs (Canon) about $140 at costco and then if you are for too extreme wide, Canon's EFs 10-22mm (About 450-500$) or 17-40mmL a little more expensive if the very wide not needed.
If you should settle for Tamron, consider checking ebay or likewise for same model you want (Consider that both exist in older and newer digital grain at about 100-150 $ difference so attention ot Xi etc that comes in lens description to compare orange with orange and not apple with pc...) where you may find many willing to sell as they are upgrading to better glass at a saving that is passed to you.
Hope my adding confusion, to your already pre-existing headache helped...
Also remember a lot of time when you happy paying less initially, you land a lot more at the end...