XP vs Standard Lisa lll, think it is mainly battery life. It is about 8 hours with my standard. Nice think is you can carry it around in your back pocket as a mobile with alo lisa harness and ipod or in a coat pocket and i think the xp would be too big for that. The LLP is kind of a must though to tell you the truth, esp if you are using it as a home amp. Originally I had your thinking and the only way they had any available was a set at that moment and I am so happy I got both at once. It is a complete home set up that way, with more access to quiet power and charges the batteries, otherwise you will be eating batteries like crazy as well.
I don't intend to carry it around too often in my backpack, just want to put it in the carry on baggage when traveling by air, without occupying too much space or being heavy. The Lisa XP is what I'm looking at, cause the set is too expensive at the moment. Will get the LLP 2-3 months afterwards when I get more cash. Although in the end its costing $150 more than what I paid for my WA6.. lol.
Another amp with a small footprint caught my eye ; the Matrix M-Stage... its long but light and small height wise, and costs $320 around. Seems interesting.