I personally like the Frontier/Woods brand RFM's. I had a 60" SD RFM and it cut great. I traded for a larger economy Frontier (I believe made by Befco) which I like but not as well as far as cut. (It does mow faster). I am a fan of the Imatch hitch, but several brands offer good models. Befco, Woods, Frontier, Bush Hog, LandPride all consistently are at the top of the list. The only additional advice is choose the highest blade tip speed you can (16,000 fpm is a good goal) and if you go with Frontier, I would get the 1060 or 1072 standard with Rear Discharge, not the Economy. They are better cutting mowers. Also, your 3120 WILL pull a 72" mower no problem, so I would get that over the 60" unless you have lots of hills or tight spaces to negotiate.
John M