Since this is in the Beginners' Forun, I will have to assume you are at least slight inexperienced.

As a general rule, fun fly type planes are built light to get a good power-to-weight ratio for vertical and 3D type maneuvers. Light weight also means easy to break if you bounce it around landing. From a repair standpoint, I would say the Somethin' Extra would be easier to fix. Sig even sells wing kits for it.

With any of the fun fly planes, it is generally the setup and your flying ability as to what you can do. The more tail heavy they are, the easier they will do most of the 3D maneuvers. Normally, you want a good bit of exponential set in. Set with the stock CG, both will be very nice airplanes to fly.

Another thing you need to be concerned with is speed. As the controls get larger the tendency to flutter increases. Most fun fly planes will not withstand a full power dive without destructive flutter. The UCD manual has a section telling you not to use larger than a 6 pitch prop.

Tight controls are a must as any slop can lead to flutter. With a trainer of Stick, for example, you tend to get a buzz when flutter starts. With the larger controls, it gets bad fast. Control surfaces can fall off and wings break.

That being said, both are nice planes. We have several UCDs in my club and had 2 or 3 SEs. A couple of the UCDs are flown by sport fliers who do a little acro and shoot touch and gos. Ons is .61 powered and while is won't climb vertically, it is a great sport plane.

The large UCD has one bad characteristic. If you are a normal new flier, you may never notice it. It has a very bad pitch down with rudder. It has some roll, too. At least, the 4 I have flown had these characteristics. One of our guys always uses rudder to correct for a crosswind on landings. He got a big surprise when the nose dropped on final. He did recover, but it scared the living daylights out of him. You put in full rudder and the plane will roll slightly, but the nose will drop a bunch.

I notice on the .46 size they have lowered the stab to the center of the fuselage. This is one of the design changes that is normally used to correct for pitch toward the gear in knife edge. I would say the smaller UCD would be a better choice of the 2 sizes for an inexperienced flier.

The Somethin' Extra is an older design and probably more sturdy than the newer, relatively lighter UCD.

If you really don't make good landings yet, one of the Sticks might be a better choice. The Ulrea Sticks are fairly light and fun to fly.