If you do a search (top right corner) on the GP Extra, you will find lots of happy pilots. Supposedly a very nice addition to the GP family of (heavy) planes.

If you want to put your effort into flying in a scale-like manner, i.e. "flying the wing" and paying attention to the lift generated on the wings when you fly, get the Mustang.

If you want to perform the aerobatics that only high-power, dedicated machines can perform ("flying the prop"), get the extra.