I don't mean to be rude but isn't this a bad question to ask online? I don't see how we're supposed to know about what'll feel best on your lap? You didn't even give us your measurements to get a relative sense of size. You should probably see if you have any friends that'll let you try theirs.
Based on myself, I'd say the 13" is definitely the lightest and fits bests my lap, albeit the working space for my hands is tiny and the edge cuts into my wrists pendent on my position. The 17" I've got right now, while usable is much bigger than the bounds of my lap, which is bad for travel and and a bit too heavy. The edge is still a bit sharp but my arms rest more parallel to the edge thanks to the rests and also gets deffered to a flesher part of my arms, making it dig in a bit less.
I haven't seen a unibody 15" in person, outside of the Apple Store, so no comment there. It's probably a fair compromise between the two issues of comfort.
Based on myself, I'd say the 13" is definitely the lightest and fits bests my lap, albeit the working space for my hands is tiny and the edge cuts into my wrists pendent on my position. The 17" I've got right now, while usable is much bigger than the bounds of my lap, which is bad for travel and and a bit too heavy. The edge is still a bit sharp but my arms rest more parallel to the edge thanks to the rests and also gets deffered to a flesher part of my arms, making it dig in a bit less.
I haven't seen a unibody 15" in person, outside of the Apple Store, so no comment there. It's probably a fair compromise between the two issues of comfort.