The UL doesn't have a double bottom, so when you stand inside it you standing on the angled hull
. Whereas the AL is double floored so it is flat. The UL is the ultralight so it obviously weighs less. Also, I believe when I bought my AL it can hold one size engine
bigger, 15hp and the UL is I believe limited to 10 or 12 but I could be mistaken.
Other than that, they are both the same hull thickness and tube size and material. Both are incredibly lite my 9AL is I believe 90lbs.