Putting horsepower aside, single is definately less maintenance compared to twins. You have double the wastegates, oil and water lines, plumbing and if one of either component fails, it's hell of of job to check probably ends up pulling both system apart. Take is as having two gf's or two wives or two mistresses.
Many drag set-up's uses single, T-51KAI/SPL but if you are daring enough and still wanted twins, then a pair of GT3040's ... Basically, if you are not a hp's freak (say below 600), keeping the original set-up would be running twins. Else, single is definately less headache and cleaner bay. Ultimately, is one setting up the car for drags or for circuit racing?
Personally, I prefer running twins coz I love headache ...