Personally, I would steer clear of the ESRI stuff. I'm working with ESRI Geodatabases and find the restrictions perplexing and restrictive. The Enterprise offering may be different, but I choose not to hold my breath. (This is a biased opinion, and I'm suggesting you do some research and make your own decision here. I have little experience with ArcServer, but a fair bit with ArcGeodatabases(SDE,FGDB,PGDB)...which I do not like)
Google Earth Enterprise Server
Google Earth Enterprise Server is a great offering. But be prepared to pay for the infrastructure and data required to drive it. A simple installation and a 15m globe data set will set you back about $(AUD)600,000 (for critical infrastructure level redundancy and some data). There was about 16tb supporting the data storage for the globe and the required resources.
Then you have user expectations to deal with. If someone sees a Google Earth Client icon on their desktop, they'll expect the Google Pro experience, not it's anorexic cousin. This is what we found, the managers came down hard on us for not delivering the full Google experience, but balked at the price tag for such a service. The solution is to buy Google Pro licenses and layer globes, with your data sitting on top of the Google Pro offering, but here you're adding costs.
What you're paying for there is essentially a fancy UI's over GeoWebCache, GeoServer on top of a Google modified Apache Server (Tomcat). Somewhat over simplified, but they are the basics.
The Google Support Folk are very helpful and will assist you as much as they can to get it working, but the raster data sets accepted for your globe are restricted to GeoTIFF and JP2000 in WGS84-LL. So be prepared to have some data processing infrastructure with that as well. For vector dataset, Oracle Spatial, PostGIS, and any other flavour of geodatabase are out of the question for vector storage. Look at Shapefile and MapInfo Tab for that. This is frustrating and restrictive.
But as @DEWright said, its great for visualisation, pretty much useless for anything else. There are things you can customise, but you'll need to get creative. The most common method for instantiating anything within Google Earth Client, was to have a Webpage that formulated your query or triggered a process where a KML was the response. This dynamic KML was then opened in Google Earth Enterprise Client, where navigation to other resources could be controlled through hyperlinks in balloons. I'm happy to diagram this to give you a better idea.
PM me for more information on Google Earth Enterprise Server...I spent 2 years working with this product to get it to do inventive things and integrate into an environment
GeoServer and GeoWebCache
Great and Free! Most people on this forum will give you some great feed back on these products. What I came across was, generally the same thing, but it is only recently that GeoServer has stepped up to deliver a tighter package.
The Styler is helpful, but buggy (at least v2.0 was), GeoWebCache was a separate feature/service on a different port(this has changed), it didn't chain WMS(it does now), and it doesn't integrate with Active Directory(may not be an issue for you, but was for my project).
Best of all it now supports WPS (web processing services). I used this in conjunction with Google Earth Enterprise Server. Google is great at Visualising data, poor at delivering it for analysis. GeoServer has great connectivity with a multitude of data formats and a great GUI for Administration.
The community is active, and consistent with releases, bug fixes and updates, and there is a great amount of information on the Web for support.
It will ingest your current KML's, as well as deliver data as KML. The reflected KML feature is nifty in that only the data is in your current view is streamed to your Google interface. This will allow you to surpass your 15MB limit....a lot. I've had a 600mb data set in an oracle spatial database be delivered to a Google Earth Client with response times around 5 seconds (using optimisation techniques in the DB and GeoWebCache)
The biggest benefit is your data is completely open. It can be served to any number of clients because it is OGC compliant.
I could go on for days, if you want more information, I'm happy to deliver, send me an email. If there is something relevant that should go on GE I'll add it here too.
Cheers...back to work now ;)