DLR easily. I like Sammy solo, but he was such a departure from what Dave was doing that it was an entirely different band. "Van Halen" means the band with DLR. "Van Hagar" is the band with Sammy.
Is Sammy a "better singer"? From a technical standpoint maybe. So are Steve Perry and Whitney Houston....both would suck as singers for Van Halen and ruin the band.
Rock singing has NEVER been about "vocal chops" anyway.
Here are some other singers who "can't sing" from a technical standpoint:
Tom Petty Bruce Springsteen Bob Dylan Vince Neil Anthony Kiedis Axle Rose Joe Walsh Gene Simmons John Lennon David Bowie Mick Jagger Brian Johnson Bon Scott Chuck Berry Ray Davies Roger Waters Layne Staley Bon Jovi Joe Strummer Elton John Neil Young Eddie Vedder Van Morrison Lemmy David Byrne James Hetfield Lou Reed Ozzy Tom Waits Leonard Cohen Johnny Cash Ringo Starr Janis Joplin Kurt Cobain Grace Slick
That's a short list. Even singers like Sammy would get kicked off of American Idol the first day.
Whatever Dave was...he had style. Sammy could never sing a song like these and pull it off. It would be embarrassing if he tried.