Instead of adding an energy system, I think it would be better to have no free version but give players a 30 day trial of Griffin AI that includes 10 days of access Dragon AI before players are split. Either:
* Given free version with energy system
* Pay a subscription fee of $5 per month for Griffin and call it a standard subscription
* Pay $10 per month premium subscription for Dragon AI.
This would allow players to test both AI's for a short period and pick the one they want the most if they decide they want to continue playing.
This would increase the games income while also avoiding the annoyance of energy systems which have a good chance of simply making users stop playing altogether as it would be almost more trouble that its worth. People come to play this to escape the annoyances of reality and relax and an energy system would just add the stress of worrying about how long you're be able to play today before having to stop.