I think you'll be fine in two days. I think we stayed 3 nights. I think you can do Maligne Canyon and Maligne Lake in one day. Both of the falls I mentioned can be as quick or as long of a visit as you want. The glacier is a few hours, but you could do that on the way out of town, depending on where you are headed next.
Which do I like better is a tough question. Banff is one of my absolute favorite places to visit (I have a third trip planned for Christmas), but I've only been during the summer once. Winter vs summer are two totally different experiences, and I haven't been to Jasper in the winter because the road from Banff is closed.
As far as a town goes, Banff is definitely better. It's bigger (but still walkable) and more charming. But the cruise on Maligne Lake is truly spectacular. And visiting Maligne Canyon is also unique (it's very different from Johnston Canyon and Marble Canyon down toward Banff). Athabasca Falls is really gorgeous and a unique formation, too.
I'd give the edge to Banff, just because I would say there are overall more activities and more things to do, but Jasper has some really spectacular features that don't compete with Banff. Two totally different experiences in my mind. I wouldn't visit in the summer without going to both places.
You also asked about Lake Louise, and I include that in the Banff area. The hike to the tea house is fun (I kind of wish I'd gone a little farther) and obviously the lake is beautiful, as is Moraine Lake, which is very close by. However, if you aren't staying at the Fairmont Lake Louise, I probably wouldn't visit LL for more than one day. There are just too many other things to do!
Peyto Lake, which is just north of Lake Louise, is also a must-see. The color of the water here is just unbelievable.
If the road is open, I would recommend a visit to Takakkaw Falls. We were unable to visit because it was too early in the season and the road hadn't been cleared, but it is the second tallest waterfall in Canada. The pictures are gorgeous.
Emerald Lake is also worth a stop. Staying at the lodge there in the winter time is on my vacation bucket list.