One of my favorite bands of all time! I have all their stuff. Some of my favorite albums of all time are 5150, F.U.C.K., OU812, Van Halen, 1984, etc.
And I prefer Sammy over DLR as a singer any day. He crushes Diamond Dave on EVERY level - technically, emotional delivery, crowd interaction, personality, talent, etc. Dave is a flat out d***. I've seen Van Halen and then I've seen the Sammy/Dave co-headlined tour. And live (where it counts!) there is no contest. Dave has lost it. He's a jerk, he has no skills, he's burnt out, he couldn't remember the words, he couldn't sing (forget on key, it was just mumbling almost monotone) and that's that. Sammy on the other hand, is just "one of the crowd". He's a fan that sings for a living. He has a fun attitude, is talented, and it shows. He has a KILLER voice, and tears it up live!
I will admit that the music made in the Roth-era was more energetic, had more attitude, were generally better songs, and was more groundbreaking. However, you can't blame that on the singers. Van Halen has always been Eddie's band. By the time Hagar came around, Eddie wanted to be pop - he wanted a stable career and chart topers. He was older, and his priorities had changed. It happens to most every band - you can't be a bunch of angry kids defying the world forever!
Have you ever heard Sammy's solo stuff? Man, it smokes! He is almost as good a guitar player as Eddie (ever see VH live when Eddie did keys and Sammy did guitar?). DLR's solo stuff was hit or miss, but it was only hit because of Vai and Sheehan. Sammy is song writer; David is a leach.
Anyway, that was a bit of a ramble. I like Van Halen from both eras, and both have their pros & cons. However, if given a choice, I'd take Sammy. That guy is amazing. One of the best vocalists around!