I think that the two groups do basically the same thing, so it comes down to service and price.
As an update, GEICO gave me a MUCH better deal than either of the above. With proof of no claims in the past 5 years, it'll cost me a whole $167/year for liability while I'm in Europe. GEICO has an overseas division that insures both cars and motorcycles, and depending on the model and year, they may offer full protection (ie collision), something ADAC didn't offer. In my case, with my 21-year old faithful sidekick I didn't want collision, but I remember that someone was looking for it...
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