Ran Monster Ovals on my 2008 Ultra for two years and 20k. Great pipes other than the chrome tips that kept degrading (changed 4 sets)
I now have a 2010 Ultra and went for the Monster Oval SlashCuts (for obvious reasons) a with 1300 miles on the clock, I think they are a little quiter, but it just might be bedding in.
If I had to choose between the Monster Ovals and the Twin Slash Monsters,it would be the Twin Slash.But my problem is choosing between the Twin Slash Monsters and the Monster Rounds.I really like the look of them both,and I'm assuming the sound is the same.
I like mine, turned the tips so the matched the bags a lil better and I cant imagine having anything other! as far as the tips getting discolored by exhaust, I use a lil brake clean on a rag and wipe them out when I give her a bath.
I actually have the original V&H Ovals as there were no monsters when i purchased mine. If I were to do it now....between those 2 options, I would go with the Twin Slash. I just think it looks better.