The G1 doesn't have 2.0... It has 1.6. Even though you can get a leaked ROM for 2.0 (or even 2.1) - it doesn't mean the G1 can handle it. I have heard of plenty of bugs with 1.6.
The Eris, has Sense UI. They are specifically building an update for Sense UI, therefore, whatever "newer" version the Eris gets, the update will be tailormade for it. Patience is a virtue. Besides, other than not having Google Maps Navigation BETA, what's wrong with 1.5?