Twins are better all around IF SIZED PROPERLY for V6/8 type engines.
NOTE: This info does not apply to 4 bangers or straight 6s/8s, as I am not too familure with whats best with them.....
A guy on turbo mustangs had I believe a single 88MM turbo...switched to two 76MM turbos, and his car spooled something like 500RPMs quicker to full boost, and two 76s flow more air than a single 88, so its peak potential was also raised.
I also recall him running quite abit faster in the 1/4 mile.
Twins also for the most part retain your exhaust note.
Singles make your car sound worse than a mid 80s chevy V8 truck with glass packs, because it combines your exhaust into 1 pipe. (think camaro sound)
Twin> single if setup properly....
only reason people run a single is
A. space
B. so they can run in the single power adder class. (single turbo> centrifigul)
C. Lack of knowledge (adverage supra owner.thinks single >twin in every car)
D. to save money
Other than that there is NO reson to go single, especially on a V8. If you want 800RWHP, you can get it with twins....and the twins will spool up MUCH faster than if you went single. (again if you sized the twins correctly...2 100MM turbos wont spool up faster than one 88MM turbo. DONT over do it)