An instructor is definately the best way to go. No chance of spending money on something you're gonna crash.
Learning on a parkflyer is also an OK way to do it in my opinion. You will crash and need to do repairs, but it'll probably be cheaper than repairing a glow plane (I don't know much about gas planes, never flown one).
I'd recomend you get the Aerobird, but as you've already got the radio it would seem a waste of money. It flies for about 15mins. Have a look at how much it is and see what you think.
I have a GWS Tiger Moth with Li-Po batteries which can fly for around 45+ minutes. Li-Pos are expensive but I highly recomend them. It's great being able to fly for as long as you like, stopping when your neck gets sore from looking up!
Whatever plane you get, I recomend Li-Po batteries.