The main differences are as follows -
1. The movement - The Motors has the 30 Second Chronograph which is unique in the Breitling range. The 6.75 uses essentially the same "big date" movement that you find in the Blackbird. For uniqueness, the Motors wins on that count.
2. The caseback - the Motors has quite a boring flat caseback with some words on it and that's about it. The 6.75 has probably the most interesting (and copied on all manner of fakes) caseback in the shape of a car wheel.
3. The case height - The 6.75 is marginally taller on account of the movement being modular and hence a little taller than the one in the Motors. Additionally, the wheel caseback adds even more to the height.
Both are great - I own both myself
- but ultimately only you can decide which you like more.
you took the words right out of my mouth. 
but between the big date and the caseback I was and still am hooked! That's just my opinion but I must agree with one of the above posts which said to look at both and one will stand out to you.