It really depends on how you are using the products. Thinktank was designed as a shoot out of equipment. Although you can use it to store your equipment while on the move, it doesn't neccesarily mean you should. The Lowepro versions, you need to zip and unzip but will provide more "storage closure" capability as opposed to thinktanks strap. I can't recall, does Lowepro have a rain cover for all their products including the loaders?
I have two lowepro gear the Vertex 200, as opposed to their Airport versions and an Apex 60AW for my G9. And of course a Domke F-3x for casual carrying around. But regarding loaders, if I was going to move around carrying my lens outside my bag, I prefer that I don't hence that's not what I will use the loaders for. My lenschangers are used primarily for quick lens changes and when I'm done shooting for the day, the lenses goes back into the pack and the lenschangers are empty. I can even compress my lenschangers and store in my bag without having to worry about having empty "shells" outside my bag. In my opinion, ThinkTank is more versatile.
If you have a backpack/bag and you need more room for your gear (for lenses), yes, you can add more Lowepro "loaders" but I would not travel around (and I travel internationally) with my gear and having a backpack and stuff (loaders) attached to the outside, is not something you want to do.
So the question is what are the purpose of your loaders? The pop-down feature is an an extension to their capability where when I'm on the move, if I have no lens in my lenschangers, I can technically use the lenschanger Pop-down to stick one my camera into it (around my neck still of course), without it swaying around. I should mention it's pop down because (it's a shoot out of changer), you can store your 70-200 and 100-400 lens with their hood in ready position.
Like I said it's not about which is technically superior but Thinktank is far more versatile.