[sassan]

I have the tamron you mentioned and am familiar with the sigma though have not shot with it myself.
Tamron is the widest, smallest, lightest lens in the world for the focal length it offers and that awesome X10.7 optical range. It does grow to very Long impressive length at 300mm that you would wonder where all these parts were hidden.

Sigma surely has much less optical range of X4.3 and I believe is heavier and longer (More space consuming).
The other side of story is that Tamron tends to be very soft (At time not acceptable in my opinion) at higher focal range, above 200mm especially with diaphragm wide open. The sigma you mentioned, formerly known as Quantaray (With the same optics and techs) on the other hand is a very sharp lens, hate to say, at times indistinguishable or very close to legendary Canon's L series quality.

I leave the price factor for yourself to figure out as I tend to find things at much lower price most people pay for so think this is a variable you need to figure out for yourself.

I look at Tamron 28-300 Di put on Canon's XT (Or 350 for non-USA) as the marathon runner photographer. The lightest smallest most powerful gadget, yet cheap with good functionality that should makes one successful photographer in Iraq, Lebanon, South America or anywhere else with turbulence, that between picture and photographers life, there is not much safety zone also considering that fact that tools can vanish at any moment. Out of this discussion but at higher demand levels, surprisingly I see the real Photojournalist even in these situations, still going for heavier 1D / 5D body and L 70-200 IS and L 17-40mm or L16-35 all the time.

I am not sure what your main need for these lenses are, but hope this helped.