roll down vs packer types with a fold over top --I use both types and the packer types work fine and the roller types have the advantage of being able to put something really long and sticking out and still being able to latch the two clips together with whatever sticking out up top. Handy for some grocery getting and or in daily use (the vast vast majority of the life use of a pannier if you commute by bike) where you can stick in stuff willy nilly like a plant, or a long baguette or whatever and easily ride (not in the rain of course)
both types do allow for overpacking to an extent, again very handy when doing grocery shopping at the end of the day when touring and going to campground or whatever.
as with all outdoor gear, top quality stuff costs more, but in the long run is worth it if you are someone who will use it for many years. Works better, will last more than inexpensive stuff. Such is life.