IS comes in handy when you are working at the long tele-end... if not just to calm the viewfinder... ever tried catching a plane at 300 mm... much easier with IS
The IS also help when you are shooting when light isn't as abundant anymore. You should use at least 1/focalpoint as shuttertime, but with IS this can be lower and still produce sharp images... Handy when it goes a bit grey out there...
Anyway... about the lens... set your budget first... because I think there are better options out there...
The EF 70-300 IS is a bit more expensive, but has a better quality to offer, it doesn't go soft at the long end as most prosumer 70-300 mm lenses do suffer from...
Less expensive and with very nice reviews, the EF-S 55-250 IS... it is stabilised and delivers very nice, sharp images...
Don't get me wrong, the Sigma's and Tamron's or older Canon's like the 75-300 can produce nice images too, but the lenses has more limites... Just so you'd know...
Don't just take my word for it...check
www.photozone.de for reviews of about every lens ( in English) and look around internetforums to get a better view on whatever lens you are going to buy...
Cheers,
Omag