Aero shaped wedge pack with easy-to-use strap mounting system.
Large openings swallow up all the things you take with you.
Gussets hold the rear flap in place when open.
Hard wearing fabric.

Strap-on takes abuse.
Compared to the Quick Release versions of the Aero Wedge seat packs, the ones that attach using straps take slightly longer to fit and remove from the bike. This is not really a problem on the whole, unless you need to constantly remove and replace it. (i.e. if you are leaving the bike locked up in a public place.)
The advantage of a strap fitting bag is that it is more durable. The quick release clip that holds the bag in place on the 'quickclip' models is very good until you have a large bag filled with relatively heavy things, like tools and inner tubes, and you ride hard on a challenging piece of trail. This is no problem with the smaller bags, but if you ride off-road and you need a large seatpack then the strap fixing version is less prone to breaking under the strain.

Features:
Expanding design on M & L models for extra capacity
Tail light strap included
1000 Denier Teflon coated fabric
3M Scotchlite reflective safety strip
Durable 'strap-on' design