"which last tends to be better for lightweight backpacking? "
I don't think one answer would suit everyone. The Performance last is meant to be a snug fit so there is no movement in the shoe at all. The foot is held solid even when traversing steep hill sides at speed. They are meant for actual racing, when you can put up with a bit of discomfort for a few hours. I have a pair of Mudclaws that are superb over steep, muddy terrain, but they start to hurt after 5 or 6 hours.
The Endurance last is roomier (by Inov8's standards) so i can wear my Roclite 295's all day.
I have a wide forefoot, so what suits me might not suit anyone else.
Shoes using the Endurance last should be more comfortable for most folk, if using them all day. Just my opinion.
I'm using a pair of Mizuno Wave Harriers at the moment, and find they suit my feet. Worth checking out if you have a broad forefoot.