There are two “styles”of T-slot aluminum extrusion on the market: grooved and smooth. In Europe the dominant style is smooth whereas in the U.S. grooved profiles have the larger market share.
It would be convenient if fractional profiles were grooved and metric were smooth, but that is a bit of a non-starter for me because I feel very strongly that the world needs to be all metric. (You may disagree with me, which is your right, but you’re wrong.)
The grooved profiles were sold as an alignment device, i.e. you could true up a joint by aligning the grooves from one stick to the next, but that’s not a terribly strong argument. You could also say the grooves make for a more pleasing appearance, but that is really in the eye of the beholder (unlike the metric vs fractional debate, where it is impossible to justify the continued use of fractional on any grounds, other than perhaps the “We’ve Always Done It This Way” argument.)
The real argument against grooves is that they trap dirt and dust, making it much harder to keep clean, which defeats one of the key benefits of T-slot over welded steel: It’s attractive appearance.
For my money, I prefer smooth T-slot extrusion over grooved, but for your money, well, let’s just say that it talks pretty loud at our company. And as grandad always said, “The customer is always right . . . even when he’s wrong.” 🙂