Luxury german might sound nicer, but might be harder to get into and to get new clients.
Most people shopping for high end BMWs or MBs have their own sales reps and don't wander into dealership clueless as to what they want. Therefore starting in a high end dealership where there might not be too many new walk ins might prove difficult unless you have your client base you can fall back on.
As for American cars, well, there aren't as many luxury choices (Lincoln, Cadillac) as there are with German (Audi, BMW, Mercedes, Porsche), but for a beginner it might be easier to start off and get hired with American brands.
Plus Cadillac and Lincoln are usually paired up with lower end brands, so in most cases if traffic is slower with these brands, you can always sell the lower end counterparts like Ford, Mercury, or Chevy and still make a decent month.
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