Please remember that this is my personal opinion; others may vary.

I went from a .46 to a 1.20; it is a big jump, but if you are careful and patient (and careful), it is the same thing only bigger (actually now the .40s seem like toys to me).

If you are serious about aerobatics and wanting to try 3D, stick with the engine in the upper recommended range, not the lower. Having an aerobat plane without enough power is just not fun.

My Yak is pretty similar to the one you listed above; I have a 1.20 two stroke in it and it's a nice setup. I would not want any less power, but it would probably fly OK on a .90. One thing that I really got used to is having power and using throttle management. Flying around at 3/4 to full throttle all the time is noisy and leaves you with such little reserve...it's just kind of disappointing. It's really nice to fly at 1/3 to 1/2 throttle and have the rest in reserve for when you really want it.

I would say of the two setups above (assuming both with a .91 two stroke), the Yak would have better overall flight characteristics but lack in vertical performance, while the Seagull plane would probably fly a little heavier and a little less stable, but have good power for verticals.

I say get the 1.20, and get a good engine for it. 1.20s fly really nice and yet are still portable. There will be a substantial difference in the way the two fly.

*note that when people talk about verticals, it doesn't mean flying a straight line up into the sky; it means pulling out of a hover, it means flying a big loop without the plane running out of power at the crest, and if you get into IMAC or pattern maneuvers, it means having enough juice to do some rolling maneuvers in your up-lines (like in a stall turn or something).