Hmmm, here is my actual experience with the two discs I have -- one eight foot 3ph fairly new and an older twelve foot transport disc in very good condition. There is no way either disc is better than the rototiller in my soil. I can control the depth on my rototiller so depth control is not a problem. Actually, I'm thinking about selling the two discs because I never use them any more. No more farming here on this old 300 acre farm, just keep the fields mowed and sell some hay. That is my experience with the soil here and it may be different in other soil.
Also, I doubt very much if your 3720 with unloaded turf tires has the traction to pull a disc.