What is the difference between a normal 75-300mm sigma lens and 75-300mm APO sigma lens? Which is better?

Question

I have recently purchased Canon 400D digital SLR and would like to go for one of the above lenses..so please advise.

Answer 1

These are old lenses. The APO is/ was a decent lens for the money but avoid the regular version. In fact, I'm not sure that either version will meter properly with your 400D. There are loads of compatibility issues with old Sigmas.
The current models are 70-300 and these should work fine. Again, the APO is decent but avoid the cheaper version.
Your 10 mega-pixel sensor WILL show the difference between a $140 Sigma, a $200 Sigma, and more expensive Canon alternative.