Cruiser bike vs touing bike vs mountain bike vs road bike?

I'm goin to start commuting by bike next month. My commute is going to be in Queens, NYC. I already planned and searched for the safest route and it will be aprox 8 miles (16 total). I'm looking for a bike right now. I'm not going to be carrying anything heavy, only a regular messenger bag. After... show more I'm goin to start commuting by bike next month. My commute is going to be in Queens, NYC. I already planned and searched for the safest route and it will be aprox 8 miles (16 total).

I'm looking for a bike right now. I'm not going to be carrying anything heavy, only a regular messenger bag. After spending a considerable amount of time reading about the different types of bikes there are out there, but I still don't know what type would be the most comfortable and appropriate for my daily commute.

I've been looking at this 10 speed cruiser.bikehttp://greatusedbikes.smugmug.com/gallery/2347548_aYdoH/1/528161815_pcxFh/Medium
The upright position seems like an advantage since I have some lower back problems. But then again, I've never ridden a touring or road bike to know how comfy they are.

Any ideas or suggestions?
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