I'd like to comment from a musician's perspective: When you change a major ingredient in a recipe, it becomes a different dish. I'm a HUGE fan of what I would call both bands, because in my opinion they have to be viewed as separate. At any given time, they were arguably one of the best bands in the world.
As a guitarist, I am a long-time disciple of Eddie's. He is my favorite. As a credential, I've played "Eruption" to many a drunk in a packed bar on any given Friday night. Correctly
My favorite DLR albums were the darker albums such as Fair Warning and Women and Children First, and those VH albums IMO have no equal. That said, 5150 was one of the most solid rock albums to come out in the 80s, and it still sounds good today.
The difference: DLR is a scatter per Big-Band and jazz definition. His "singing" is laid out in meters using syllables as notes. Listen to "Hang 'Em High" as a reference. This is what makes sometimes nonsensical lyrics sound so good. The intrinsic reason DLR fits so well with VH is because Eddie plays so late in the measure, or "deep in the pocket", that it forced DLR to sing directly off of what Alex played, which is essential for scatting. This also made them sound "heavier", because it drives the song along. This is why it's difficult for anyone to cover DLR's songs. What was the point of "Big Bad Bill is Sweet William Now"? To illustrate that the guy thinks he's a scatting Vaudeville act, which he clearly is. The only VH song I can thing of Sammy did as a scatting type song was "Human Beings", and he even admits its crap, but that's because he can't scat. Does being a scatter make DLR better? Just depends on your taste. His style matches his arrogance, which is definitely part of his cool factor.
As an alternative, Sammy is a true singer in the classic rock/ 80s vein. He's a more open-aired and melodic singer. This gives more room for writing tunes that actually have hooks such as "Best of Both Worlds" and "Summer Nights", which is the reason they enjoyed better album sales when Sammy joined. Some people didn't like it because they felt VH "sold out", but the fact of the matter was that their songwriting just became more culturally relevant, and Eddie was able to explore keys with someone that actually had a singing voice. You can usually tell that there's a point to the song with Sammy's writing, whereas DLR's songs are ambiguous at best. Compare Sammy's solo work to DLR's and you'll get the picture, as DLR's solo work was extremely weak. Sammy can write and play by himself, whereas without Eddie and Alex, DLR has very little to offer.
All that said...
I saw VH with Sammy a couple of times. The first show was in 1989 when they were on top and it was one of the best shows I've ever seen. Check out the dual guitars on "One Way to Rock" on youtube for reference. I saw them again with Sammy maybe 4-5 years ago for the "Eddie Bail-Out" tour. I've never been so disappointed, but by this time Eddie had become a crack-head.
I saw VH with DLR just before the thing fell apart, maybe 2 years ago. DLR was brilliant, his delivery was excellent, and I got more than I had hoped for. He was clearly on his game. Eddie's playing was even pretty good. BUT, because Eddie is so childish, he didn't bring Michael Anthony along. That means I had endure his pubescent sloppy teenager playing as sloppy as he looked, complete with pants hanging off his backside. He destroyed every other song he played on. It looked like they'd picked up a kid at the local arcade and stuck him on stage. Not what I'd call a reunion, but glad that I finally go t to see DLR sing those old great songs.
Someone mentioned Cherone. I did get to see Extreme play at the Fox Theater in Atlanta for the "Three Sides" tour around 1991. They were awesome. Cherone is a great singer, bad fit for VH, but this was a douche move orchestrated by a band manager that was playing to Eddie's vindictiveness against Sammy. Cherone had nothing else to do, so why not.
Oh, and Sammy's tequila is GOOD