What speakers are better to use for left/right with a home theater projection screen?
Between the two you offer, I would go for the floor standing model any day. But the other poster is correct not all florstanding speakers are better than a bookshelf, even within the same brand (SONY, JBL) but B&W is real good about this.
Can I replace the front speakers of my Home theater with larger floor speakers?
yes, but you should consider changing your center speaker to match. i wouldn't recommend you use them in parallel. you could use the extra speakers in a 7.1 surround set-up.
which speakers are better?
Lol at some of the answers. Bose?? Lol that company needs to get banned from earth. HK makes good speakers in general. You can never go wrong with them. Altec lansing, lol, not bad but I would never buy them. For the price of expensive computer speak...