The biggest benefit of the other unit is its ability to fit in the dash no modifications. For most people this "feature" alone would be worth $100 to $200 dollars.
Compare a 7" regualar screen at $200 and a 7" flip out screen at $250 and an indash for $450. The indash is the same thing just easily put in the dash. And the price reflects that. So I would go with a 6.5" screen. You probably can find a better screen at 6.5". Maybe even 6", but that would push the low side.
If you are doing a large scale, then maybe contact samsung or a similar manufacturer directly and see if they currently make any 6.5" screens for some application that could be easily adapted to VGA. Assume they want a large batch order upfront though!