Bret had the NWO welcoming him in a segment on Nitro. Luger was welcomed by The Brain (good) and was saddled with a terrible gimmick (not good). Bret's intro was better.
Bret won gold in WCW. Luger did not win gold in WWF.
Off the top of my head I can only think of two significant Bret Hart WCW matches and one of them involves the concussion. With Luger I can think of WMIX, RR94, WMX, and that dumbshit Summerslam main event. Even at whatever age (12?) I couldn't wrap my head around this man celebrating a countout victory in a title match when the stipulation stated you can't get a rematch. So by the metric of sticking in my memory, Luger had a more memorable stint in WWF than Bret did in WCW (And Bret's was more recent, though I was busier in that time too).
Bret got to fuck with Hogan, Flair, Goldberg, Sting, and a bunch of other big names. He also got to tag team with a few big ones too. Luger feuded with Mr. Perfect (good), Yokozuna (good), Tatanka (I liked him when I was a kid, not sure how he'd resonate now), and then teamed with British Bulldog in a team that had awesome theme music and was otherwise unremarkable. Also, he attacked Bret @ The WMIX presser and then...nothing? He was a face a few months later. Maybe they had a house show run?
Hell, am I remembering correctly that Luger wasn't even supposed to wrestle for WWF and he only went there b/c the WBF tanked? Bret Hart at least signed with WCW to wrestle for WCW. Even if his run wasn't the best thing in the world, at least his consolation prize was the money and not because the project he was with failed. Granted, I guess that means neither Luger nor Bret wanted to be there.
Anyway, I vote Bret.