I believe that MOLLE is an attachment system, ALICE is an lbe system. You can have ALICE packs with MOLLE.Not really sure if you want answers about backpacks or the attachment systems themself.
I'm going to write about the attachment systems. Being a reservist I have hardly ever actually carried the packs on my back for a extended amount of time.
I believe the MOLLE system to be the superior. If you interweave a mag pouch to a vest the way it is supposed to be, the strap can come unbuttoned and the pouch will remain snug to the vest. The same can not be said for the ALICE system. Alice clips get bent and come undone. When this happens a canteen full of water becomes a aggrivating flopping piece of equipment.
If ALICE equipment is all you have, or all you can afford, then by all means use it.