Treated my self to the Ortlieb Bike-Packer Plus, as I prefered the easy access, to in and out of the bag. The equipment is really good, total water proof, I had a nasty fall at 32 mph, rolled with the bike several times and eventually stopped with ,me still strapped to the pedals, and the bike upside down. The bags were still in position, did not even need to re adjust anything, and I thought to my self if the bag had half come off, it might have got caught in the on comming traffic, perish the thought . As for the compartment bit, i thought this might pee me off a tad, but now I prefere this as I open the bag and everything is in one compartment, easier to find what you are looking for, It also makes a big difference to be able to not pack in many bags, as this in itself takes up more room. As the bag is totally waterproof, you dont have to pack things in bags. The easy on off system is great, if you are worried about some one tea leafing your kit, this can be secured to the rack via a lock & cable. As for the Arkel, really can not comment as I never owned one. As for the covers for the Arkel, well that taking up more room to store these, does not make sense to me..

good luck