RE: Yak vs Extra
The Brand / Design will make a huge difference. What model Extra? Did you have a particular plane in mind?
General Characterizations
Extra- Clean flying, Tracks great, great snaps, stalls a little quicker, little push inverted usually
Yak- Draggier, constant speed, bigger (more cubic area), Floats a little more, very true flying, docile, Tends to dig into snaps easy, usually hands off inverted