There's really no reason to wait, as long as you keep a few things in mind.

First, when the second plane is done, it doesn't mean you're ready for it.

Second, If you crash the trainer, repair it. Don't use the addage, "Well, plane #2 is ready to fly, so I'll put the equipment in that one."

Third, If the trainer gets totaled, go out and buy Plane #3... ANOTHER TRAINER! Get an ARF that will get you back into the air quickly. It doesn't take a genius to figure out that if you can't handle your trainer yet, you're not ready to move up.