As far as I know the only major difference is really the case colour.
I have a GE Imager 901E which has a slightly higher scan rate and an additional point convergence board. The tubes are the same as the 9PG (not the extra).
This is one of the best 7" CRT's available. It can easily out perform many 8" projectors due to its very tight focusing ability and colour corrected lens filters. If you decide to look at one make sure you view the crt surfaces with lenses removed and the projector switched off, as this may be the only way of seeing wear on the tubes. You should also have a read over on avsforum for more details as there are some real nec experts over there!
These projectors are not as easy to set up as some but your patience and perseverence will be rewarded by a superb image when you are done. Ideally you should run the projector line tripled ie 1280 x 720 pixels for truly cinematic results.
You can check the number of hours by pressing the "display" button on the remote. This will display a menu with around 7 items in it. down towards the bottom of the list you wil see "HOUR METER".
The hour meter on its own can not be taken as indication of tube condition. If the tubes have been run with the contrast too high the tubes will wear prematurely. When I bought my machine it only had around 2000 hours on it but the tubes were showing signs of wear and had to be replaced because in my setup I used far more phosphor than the previous owner which resulted in brighter edges on my screen.