I have a bakflip and I am really happy with it. Here are some things to consider:
Pics and review of Bakflip
Pics and review of Retrax
Price:
Bakflip between $500 and $600
Advance between $500 and $600
Retrax between $800 and $1000
Utilitrack -
Bakflip - requires flipping at least the forward section of side tracks. (flipping track will prevent using the sliding accessories on that section without some modification and can also be a pain in the @ss because the bolts holding utilitrack sections in place are covered in loctite)
Advance - uses utilitrack to mount. prevents moving sliding accessories from front to back without removing cover mounts so accessories are basically limited to a small section of track
Retrax - requires removal of front section, not sure how it effects use of the side tracks or sliding accessories (some have reported problems others say not an issue)
Access -
Bakflip - cover can be kept folded up against cab (blocks visibility), or 1, 2, or all 3 sections can be folded down. You must always access from the tailgate forward...there is no way to open the front section while the rear is closed. When folded up against the cab you might lose 4 in. of space at the top of the bed rails. Easily removes with 2 bolts. Must lower tailgate to open cover if fully closed, tailgate serves as the locking mechanism.
Advance - can open 2 rear sections or front section but cannot completely uncover bed without removal. Easy removal I believe similar to bakflip with only 2 bolts. You can access front of bed without uncovering rear. Has locks integrated into the cover which allows you to open it without opening the tailgate.
Retrax - Can lock at any position along its track but the canister takes up about a foot at the front of the bed. Removal is difficult (takes 2 people). I am pretty sure you can open it without opening the tailgate.
Appearance -
Bakflip/Advance - black plastic somewhat similar to bed rails
Retrax - shiny black lexan
Other -
Rollups can potentially freeze up in heavy snow/ice. I haven't heard any complaints about this with the retrax but my friends Pace Edwards does.