I have had both of these planes also. Depends on what you want, how much room you have and so on. The larger plane costs a little more to build, I recommend a OS .91 FX or ST90 or Magnum .91 XLS for the 61 size. A magnum .52, st.51 or OS .50 SX are great choices for the 46 size. For both you may want to upgrade servos from the get go. If you like Futaba, the 3010's are reasonable and will do great on the elevators and ailerons, the 3305's work great on the rudder and std servo or mini servo for the throttle. If you are JR, the new sport servos are terrific, especially the ST125mg's. Of course you can use these, with Futaba also. I would recommend them all the way around for the .61 size UCD anyway.

Both planes have a weak landing gear mounting area in the fuselage. A couple of hard landings and it will break out. You can wait for that to happen, or you can remove the covering and put some glass cloth over that area of the fuselage at the beginning.

The planes definitely fly different, the .46 is inline on the wing and horizontal stab and has those tendencies that come with that setup. The .60 size is not inline, but is above the wing axis and has those tendencies. Most folks like the .46 over the .60 for it's quick response and 3d agility. I liked the .60 because it seemed to have better, almost pattern like tendencies on low rates, was a little slower and more graceful.

For a second plane, you can do worse. Just start out slow and easy on slow rates with about 25-30% expo. You can program out the few coupling issues there are as you get used to the plane and learn more about aerobatic trimming. There are some really great threads on RCU about inflight trimming for aerobatics.

Good luck with your UCD, hope you enjoy yours as much as we do ours.