Interesting s my dad has that exact same Gravely and my uncle has the exact same Ferris, having run each I have a few good comments.
The Gravely is a beast, built like a log wagon and rides like one too. No kidding it is built VERY well,has very responsive hydro's and a ton of power. It rides rougher than anything I've ever been on and I don't care one bit for the cut nor the way it disperses clippings. I have run dads Gravely several times and each time I dislike it more and more. I literally dislike it to the point that when my Grasshopper was previously in the shop for a few weeks I just let my mowing go. He runs a small commercial outfit on his own and occasionally I help him or do them when he can't for some odd reason or another. He had a Toro previous to this Gravely and I loved that one, I did alot of mowing with him then because I also had a list of accounts and I used his mower, which in turn he used me on his accounts..... it was a good deal for the both of us. 2 kids later and working full time I don't have the time to still be doing it.
My Uncles Ferris. Cuts nice, disperses clippings well, rides nice. I don't really have any complaints on it but I haven't run it as much, only twice.
Myself I went with a Grasshopper front deck. Occasionally I'll mow with my dad if he gets behind due to rain or we have to get all of them done quick, but mostly it is an on the farm personal use machine to do about 3 hours of mowing a week. I love this mower, somedays I want to just sit on it and mow and find more to mow just to get away into my own little world. It rides 1000 times better than that blasted Gravely, cuts well and disperses well. The only downside to the front deck for me is when mowing when it is windy if you are discharging into the wind it will likely blow grass and dust back onto you. The uphill traction people complain about I have not found to be any better on any midmount I have been on. Going backwards I do believe the Grasshopper would climb a cliff, sidehill traction is great.
Of the two mowers you listed I'd go with the Ferris, but then again I'd go with a lawn tractor that takes 3 times as long to mow with before that Gravely.