Hi Hzgray. I've been researching this question the last couple days. I ran into a situation where I was able to get a BIG discount on the Ariens Apex 60" with Kohler 7000 motor. My immediate thought at the time was buy now and research after because the order can be cancelled.
At this point, I've been researching online and contacted Ariens directly. I spoke with a sales rep and they did a comparison with the specs they have and concluded that the Kohler motor on the Gravely HD is 26 HP and 25 HP on the Ariens Apex for the 60" deck version. The HP for the Kawasaki motor is the same on both mowers for the 60 version. I personally don't think you can go wrong with either motor. Most folks will typically choose the Kawasaki, but the Kohler 7000 has proven to be a good, reliable motor.
Another difference is the paint which is obvious.
One thing i seen in my research is that both mowers mention having a tubular frame. Gravely has the HD listed as a fully welded tubular frame. Ariens has the Apex listed as a fully tubular steel frame. I've not found out yet if the Apex frame is fully welded like the HD.
Everything else between the two is the same from my research so far.
The only thing that scares me about Ariens (particularly the Apex series) is that I've not seen many customer reviews. On the flip-side, I see a lot of people raving about Gravely. All mowers have issues at some point, even Gravely. Just check out this website as a start.
I hope others will chime in that may know more about these mowers.