Based on holding, wearing, and examining the Prodiscus and Grip EQ bags side by side, I can say that the difference between the two is significant. The Prodiscus is not simply a bigger Grip. Yes, it is bigger, but it's a completely different bag. I cant comment on how comfortable each bag is during and after a round, but I should be able to do so after this weekend. I didn't notice a big difference when putting it on my back initially, though.
The Prodiscus has more storage. That's the bottomline, I think. I would say you could easily fit 35+ discs in the Prodiscus. That's crazy. Discs fit in the side pouches of the Prodiscus (3 each), the vertical storage pockets (2 each), the main compartment (24), and the top putter pouch (6ish). The Grip fits 20-22 for me, max (17-18 in main compartment, 2 up top). The storage of the Prodiscus also equates to a significant more storage space for items other than discs, though. I can easily fit a warm XXL jacket in each of the vertical side storage pockets as well as the putter pouch up top. I really appreciate that, but all I need is one spot to put a jacket in addition to my discs. I do not, however, see a need for the two side pouches that hang off of the bag. Those are what make the Prodiscus bag storage overkill (for my needs).
The bottle slots on the Grip are dedicated and are seemlessly integrated into the side of the main bag. The Prodiscus's bottle slots are less dedicated, yet still functional. More than half of the material holding the bottles in place is actually part of those bulky side storage pouches and the vertical side storage pockets.
The materials feel just as durable and the zippers are fine, so that's good.
The bottom of the Prodiscus has flexible rubber bars instead of rigid bars, which means it may sit on uneven ground with more stability.
They both feel the same when you pick them up by top strap, which I find myself doing during rounds.
The Prodiscus's rainfly is attached to the bag in its own dedicated, stealthy pocket up top. The Grip's is free standing in a stealthy pocket behind the bottom lip of the main compartment opening.
The Prodiscus has more storage slots and zippers in the vertical side pockets and uses softer material in some spots. The pocket directly above the main compartment has that soft material making it perfect for a cell phone and a camera. The Grip has the same hard material inside and out of every pocket and whatnot.
The flap on the main compartment of the Prodiscus is shoved behind the discs and stays in place naturally. There's no problem pulling discs out or putting them in. The Grip uses velcro to keep it in place. No problem pulling discs in and out.
The top/putter pockets are really different. The Grip's is smaller and has a useful button strap that elevates the discs out of the top for easy access. The Prodiscus's is BIG and you have to reach in to grab the disc. I put 3 midranges and 3 drivers in there easily. It doesn't have any mechanism to elevate the discs.
The Prodiscus has loops at the end of the draw cords that are used to tighten the main shoulder straps. Both bags have straps that clip across the chest. They also both have straps that traverse the vertical side pockets.
Those are the main differences and comparisons I can make between the two. I'm on the fence about which one I will keep. Thankfully, I can still send the Grip back to the manufacturer (I'm within the 30 days still). I don't think reselling the Prodiscus would be that difficult.
