Equipment what lens? (beginner)

Sorry to thread hijak mate - been looking over the lenses recommended above and have also been looking at the Tamron 28-75mm f2.8 SP AF.
Kinda torn and not really sure what to do, currently using 450D kit + Sigma 70-300 tele (non APO) and looking to replace the kit with something a bit quicker & better
Quite tempted by the Canon 50mm Prime (nice and cheap so won't break the bank) but can't decide what to replace the kit lense with. The 28-75 Tamron and the sigma (above) don't leave me "stuck" between 50 and 70mm (not that that's necessarily a bad thing i spose, but its nice to have all the range covered) but the 28-75 leaves me lacking a bit at the shorter end.
I guess the question is, is it worth more to have a fixed aperture or bit more flexibility in terms of length....

 

The Tamron 28-75 is an excellent lens but is better suited on FF, IMO. Then again, if you hardly ever shoot wider than 45mm the lens is a great choice. The Tamron 17-50 is sharper than the 28-75 and while there is a significant gap between 50 and 70mm I wouldn't call it a deal breaker--OTOH it all depends on you.