I have a Deere GS30. While I like the machine I would probably go with the Exmark. I have never used or worked on an Exmark but I have a Scag. I think Exmark and Scag are pretty similar and I like the Scag more.
Here's why- you can buy a much wider selection of parts, both OEM and aftermarket for the Scag and I assume for the Exmark. You can't for the Deere. 2 blades for the Deere cost me about 25 bucks and I didn't have the option of high-lift, gator, medium lift, etc... Deere blades are "keyed" which means they have a special hole pattern to fit on the spindles.
You can only buy parts at a Deere dealer. You may be able to get simple things at other stores but nothing too extensive. My Deere dealer tried to get me to buy "their" engine (Kohler 13) because it was the only one that fit. Well, I took a chance on a Kohler 15 from my dealer because it was $250 CHEAPER and it fit perfectly. So I feel the Deere people were trying to hose me.
Make a long story short- I have spent the SAME amount of money to refurbish my Deere as it took to COMPLETELY rebuild (including paint, bushings, spindles, wheels, blades, everything) my Scag.
The main thing is the cost of parts in my opinion!
The Deere cuts better but I'd pick the Scag.