The question is, what is the list of skills that a trainer should allow you to perfect? What are the signs that you need a different plane?
Sorry I don't think I directly answered your question...
1. Take-offs - be able to keep the model tracking down the center line and perform a realistic climb out (i.e. don't yank it at a 90 degree angle)
2. Pattern & Truns - be able to consistenlty hold an altitude while flying the pattern especially when turning.
3. Mild aerobatics - loops shoud be round and start and end at same altitude. rolls should not drastically alter models flight path for example gaining and losing altitude while rolling is a no no
Inverted flight should be entered and exitied at the same altitude.
4. Approachs & Landings should be consistent and use proper amounts of throtle and rudder (when required, you should also know when to use rudder) Throtle management and control are the marks of a goood landing. Practice, Practice more practice
5. Confidence when your confident in your ability to fly your airplane and Know its particular behavaior.. ITS know time to do it again at a lower Altitude!!!!
Alot of people complain that loops and rolls and inverted flight with a trainer are boring... If your bored try a loop that starts and ends at 10ft. Try some low inverted passes.
When you have the skills to do this consistently I think then your more then ready for that second plane... Then the fun begins agian because its time to learn the next models particular flying characterics... and the next and so on!!!!
BTW I personally think making consistently good landing on a trainer in various wind conditions is harder then doing it with a "higher performing sport model"