I have the P99's they are very good quality. I also have Arkel bags. If I was to do it again, I would get all Lone Peak, the Arkels are too heavy. I have the GT28's and the GT54's - the 28's are fine, but the 54's are covered with stupid gimmicks. I just wanted some pockets, not all the silly removable flaps. Next time I'm getting the LP 400's for the rear bags.
The L.P. zippers are a little lighter weight than the Arkel Zippers, but I haven't had any trouble with them. They've been on about 8,000 miles of touring, and are in good shape still. A squirrel did eat a hole in one of them - but I doubt that would have been any different with Arkels. Ortlieb may have kept the squirrel out.
+1 to TheTouringStore. Also, the L.P.'s have a hook that clips over your rack, so decide on the rack first, then you can get the right size clips with the bags.