Haven’t done either with loins, but my two standard tenderloin preps are:
Lots of grainy mustard, lemon juice and zest, garlic, olive oil, black pepper, and lots of fresh rosemary. Should be a thick paste that coats the meat. Best grilled.
Soy sauce, toasted sesame oil, garlic, ginger, white wine, red pepper flakes. Equally good grilled or roasted.
Soy, Oyster sauce, Hoisin, touch of fish sauce, sesame oil & star anise. Boiled after being used as the marinade it creates a delicious glaze for the pork with rice & long beans.
I had a fennel seed crusted pork loin at a restaurant last summer and it blew me away – kind of like a pork sausage flavor. I’ve had a lot of different pork loin preparations over the years and that one was the best.
One that I’ve done in the past involves brining the pork loin first, then a light application of olive oil, and then a generous sprinking of fennel pollen, with a little S&P. Yummy.