I think you should buy a Hybrid based on your description and here is why...
Screw that. It's your money and if YOU want a mountain bike then buy one. It's unreasonable to expect an LBS to have every single bike available in a particular manufacturers line up available for test riding. However, if you're talking two bikes in the same family, lets say a Trek 4300 and a 4500, the 4300 uses the same frame with the main difference being graphics and components. The shop should have no issue ordering you the bike you want if you test ride the bike with the same frame and decide you want a few models up the chain.
If that isn't what they were doing, take your money elsewhere.