I have the 32 qt sideways rack and my room-mate has the 32 qt narrow rack (I assume the differences would be similiar scaled down) so we have seen first hand the differences between the two.
Narrow Rack
Takes up less wall space
Harder to pull drawers out (we often splash the water)
Less Room width wise with tub divider (harder to fit a hide, only applies to 32 qt)

Wide Rack
Takes up significanly more wall space
You can see your snakes better through the "window" (which I like)
Easier to change 1 32 qt into 2 15 qts

We have found that temperature issues are the same with both setups. The narrow racks don't have as much space covered with heat tape, but they have more insulation from the walls, while the wide racks have more floor space covered with heat tape but not as much insulation from the walls. It has been our experience that these qualities even out.

Hope this helps!