I have a lot of experience on both machines, the darkroom I teach in has 3 of the D6700's, and one 6600(plain black and white head). and I have the 670XL in my personal darkroom at home.

You would want to hedge your bets on the 670XL, it is the better machine. LPL build quality is top notch on both machines, but the 670XL has certain advantages:

Longer center column for bigger enlargements
Ability to easily spin enlarger around on base for floor enlargements
Allows for tilting of the head, and tilting of the bellows for perspective correction or wall enlargement
Has a larger standard baseboard for a working surface
There is a locking switch for negative carriers which I like

Advantages of the 6700:
Very stable, and keeps alignment well as there are not many variables for adjustment of head
Raising and lowering the head is very easy, you just squeeze on column lever and push up or down, but not too precise
You don't need a separate power brick on the side

I would advise you to get the 670xl, hunting for carriers is a pain, and the 50mm 2.8 lens is optimal for 35mm