As others have commented, without mid-fork mounts you want the bar over the front. Otherwise a rack held on with a U clamp does not have a lot of structure preventing it from shifting in and out.

If you are hitting rocks and such while on the gravel, I am not sure how well a U clamp on the top of the rack will work if the weight of the pannier could pull the U clamp down. But the Blackburn style where the top of the rack could not slide down because the bottom of the rack is attached to the fender mount at the bottom may be better. The design of the rack means that the weight of the pannier would in part be supported with the fender mount at the dropout. A Tubus Tara however has a top bar that would need to clamp onto the the fork blade and only friction from that clamp would hold the top bar in place.

I used the non-hoop style (photo in a post above) only because I wanted to make it easier to pack in an S&S case where I did not have a lot of room. My other bike that is not an S&S coupled bike has the hoop style rack, a Tubus Ergo.

I put my really dense stuff like tools, spare tire, etc., in the bottom of a rear pannier. If you are on gravel, you may find that less weight in your front panniers makes your steering less heavy if you are steering around obstructions. I try to get the center of gravity of my panniers as low down as possible with the lightest stuff like a sleeping bag on top. I think front loading panniers makes more sense on pavement where you are going faster and less likely to have to steer around obstacles.