Installation is certainly harder but I think you have it right in that the efficiency is the biggest impact. However, that could be a positive in lower sun elevation angles so you might end up with a more extended season than if you had a flatter roof.

BTW, I just converted my 1 HP pump to a 1/2 HP two speed and the solar panels prime just fine at 25' so your 1.5 HP should do fine on the second story roof.