I've flown the Seagull 40 sized Spacewalker, it flies nice but nothing special. It's wing loading might be slightly high for a second airplane.

An Ugly Stik is a very good 2nd airplane, people been building them for about 45 years now, so that should tell you all you need to know. Either 40 or 60 sized fly great, and are easy to maintain. Just depends on how much fuel you want to burn.

The Pulse series fly extremely well, but won't take as much abuse as the Stiks.

To judge the "next airplane", look at the wing loading and look at the wing design. You really want to stay with a constant chord wing, and a relatively light wing loading. The rest doesn't matter too much.

Now if you started with the typical 40 size trainer, it had a wing loading of 16 to 18 oz per sq. ft. (unless it was a pig). I would suggest that for a more advanced "2nd airplane", that you limit the wing loading for another 40-45 sized engine to the 18 to 22 oz range.