I've had and used both. I used the Ohm for over a year but only used the Windrider for a trip or two before changing to their Porter model.
IMO, the Ohm compression system is not that good, and the materials are prone to failure. The system on the Windrider is more traditional and works just as well, if used correctly.
The exterior pockets are fairly comparable but the material on the Windrider will be more prone to snagging if you plan to go off-trail. The large holes in the mesh are likely to get caught in brush. I prefer the mesh on the Ohm, but HMG does offer a model with fabric pockets.
The shoulder straps on the Ohm were initially a lot better than those on any HMG pack, but this has been fixed in recent revisions. I had one of the initial Ohm runs, so compared to that the latest HMG hip belt is considerably better.
The Windrider pack fabric is quite durable and 100% waterproof. The pack itself won't be waterproof unless the seams are sealed, but mine was submerged for what felt like 20-30 seconds back in April and didn't let in more than a few drops. The pack fabric on the Ohm is initially waterproof (more or less anyway) but the PU coating will wear off over time. The Ohm pack material will also absorb more water. Of course being waterproof and absorbing little water may not be important for your intended use.
Both carry about the same, assuming you get the sizing correct. IMO, the load lifters on the Ohm are unnecessary, but they do work. Brian Frankle was an excellent pack designer and it's a real shame he got burnt out.
Honestly, I don't feel like you can go wrong with either. Both have perks and pitfalls, so just get whichever fits your individual needs better. Since I've gotten a packraft and started guiding more, neither fits my needs now so I moved to the HMG Porter.
Strictly looking at ergonomics and anatomically correct harness design, the GG packs aren't in the same league as the Ohm or Windrider.
Brad - if you happen to be in the Asheville area I can let you check out my Porter. I can probably borrow a modified Ohm (a lot of the fabric has been replaced with Cuben) for you to look at also.