Specific to training my opinion would be go with a .40

That is the least expensive size because it is the most common, so there are economies of scale from mfg to sales shop. Servos are $14 each, the .40 LA is cheaper than engines smaller or larger. More good trainers available. Less investment in materials.

If you are independantly wealthy a gas plane would be wonderful. Big and friendly, no field box to haul (no starter or glow plug driver needed). Strong and less wind shy.

But a momentary lapse of control and $800 to $900 is a pile of sticks even with the basic airframes.

And when you have your "learning experience" with a finger in a prop it's probably better to start with a 10" or 11" one instead of a 16" or 18"