Either one is a great plane. The four start 40 (4*) is more docile and even though it is aerobatic, the Something Extra (SSE) is more aerobatic. The SSE can slow down as slow or slower than your Kadet does and for that matter so does the 4*.

The SSE has a nice easy roll rate on the lowest throws that the book gives you but turn the throws up to the book high and the roll rate is blinding, forget about trying to count them.

Both are true point and fly planes which means the plane will not try to recover from any attitude until you tell it to. The Kadet will try to roll to upright if inverted or steeply banked.

Either one will make a nice second plane so ask yourself, do you want a nice docile long wing trainer or if you can fly the snot out of your Kadet then go after the SSE.