The off the shelf 6112's will give you the correct stroke where as the spacer will give in increased down travel.. Its that simple

I use the TT spacer because I wanted to clear bigger tires before shelling out on a serious off road setup (I hit off road parks and trail ride once or twice a week). If you never plan on doing any hard wheeling just run the spacer and call it a day. When the pro factor units give in, just get the 6112's with adjustable perch and use the spacers as a paperweight.

I wouldn't think to much about it lol... Congrats on your Pro purchase, welcome to the family :-)

EDIT : There is also a difference in valving between the 6112 (off the shelf) and the Pro 6112, the latter is designed specifically for the truck.