I think one thing to consider is how much do you like your trainer? If you find flying the trainer is fairly challenging then I would stick to a "milder" 2nd plane. However, if you are getting really bored flying circuits with the trainer, I would suggest a sportier plane.

I just started flying around 4 weeks ago and have already bought, assembled and flown my 2nd plane. My second plane is a Great Planes Extra 300S .40 size - I have a .46 engine in it.

Set your dual rates so the low rates are around 50% and the Extra 300S will fly similar to your trainer. The biggest difference is that the trainer is fairly self-correcting vs. the Extra will keep going at the same angle of attack until you change it. Landing is also a little different in that you need a little more speed on a taildragger - but not much for a 3 wheel landing.

The cool thing about a sport plane as your 2nd is that you can keep turning up your rates to make it more aerobatic as you get used to the feel of how it flies.

Good luck, and have fun.