Once again I find myself at odds with some opinions. Since you say you are not looking to do aerobatics, the Cub could be a good second plane. Certainly a better choice than an Extra. About the size, I've flown .25 size planes for years. I don't consider them to be "small" or twitchy. Last time I was at the field I was flying my OS .32 powered World Models Aircombat Zero with a Rossi .53 powered Great Planes F-15. While the F-15 was definitely faster, my little plane actually kept up pretty well.

You didn't say what size your trainer is. I'm guessing that since you are looking at a .25 size cub, your trainer might be a similar size. Personally, this is the size plane I prefer. They are small enough to fit in a car, don't use too much fuel, and are still plenty stable and capable in the air. Ignore the "bigger is better" crowd who try to convince everyone that .25s are twitchy.

Anyway, about the cub. I don't have a cub right now. One of the planes I fly is a Modeltech Extra 300. It's my favorite plane right now. If the cub is of similar quality as the Extra, I'd say go for it. In fact, when I went to look at the link you provided, I'm wondering if I should add this Cub to my fleet.

Assuming your trainer is a trike, you will find the ground handling of a tail dragger to be a bit more challenging. The toughest part is to get it to go straight, and keeping it from nosing over, when taking off and landing. I say ignore any nay sayers who try to steer you away from a tail dragger. Hopefully the tail wheel on the Cub is steerable. My trainer, a Sig Scamp, was a taildragger powered with an Enya .15. I consider my reflexes to be pretty average. If I could learn on a taildragger, I think most people can learn to handle one after becoming comfortable on their trainer.

You didn't mention whether you already have an engine that would work in the Cub. If you don't, you will find many people suggesting a 4 stroke. I agree with this recommendation. There's something magical about a 4 stroke powered cub circling the field, doing touch and go's, etc. If you've only got a 2 stroke, that's ok, just be sure to choose a larger diameter, lower pitch prop, and of a size so the engine doesn't wind up too high. This will keep the speed down, but give plenty of pull.

Good luck