While I think it is a matter of personal preference I've found, for me, that using card loaded with the brass down is much faster. I tend to "combat" load the mag tube in that I keep it topped off as I shoot. Shoot 2-3 shells and load two without ever looking or changing the guns focus on the target. But this is a technique that takes practice. To facilitate this I've "smoothed" out the edges around the magazine loading port which helps you thumb in shells much quicker.
Now there are other ways to load your cards. Many folks load their most forward shell with brass up to facilitate reach over single ejection port loading if they run out of shells in the tube.
Others will load the rear couple of positions on the cards with slugs (assuming they are typically shooting 00 buck). Some like these brass up so they are easy to recognize while others simply load them brass down like all the other shells and know by feel which shells are different. These methods are used by many who live in bear country and always carry a couple of 3 inch magnum bear loads.
Bottomline - you really need to develop your own technique. Load up some cards different ways and practice HD or operational (non range) shooting to see what works best for you.
New shotgunner here. I’ve been practicing loading/ unloading with dummy shells, and I’ve tried both brass up/down in the shot cards. Up just seems…unnatural. Brass down - for me - makes for a smoother transition from card to mag tube. Maybe if someone has a fixed mount side saddle that’s worn such that shells inserted brass-down fall out, then brass up would be mandatory. I use ESSTAC shell cards with the Velcro back.