I was pondering over the very same question.
Having test ridden them both, it is a close call. The SL is a more versatile bike if you put tyres other than Schwalbe Kojak on it and it can be even used for trails to some extent. The TT is a pure road bike. The extra 1.3kg or so is more than compensated for by the wider gearing range. The TT ranges from 28" - 125" whereas the Mu SL has a range of 32" - 93". The higher gearing on the TT combined with the drop bars will make it faster. The extra weight will not be enough in this case to slow it significantly.
Having said that I opted for the Mu SL I still find it very fast and as Nekohime says it is well capable of leaving many bikes in its shadow. They both come highly recommended. If I wanted to do a century the TT would be my choice. For all-round fast cycling the SL wins.
By the way the TT may well be discontinued in 2009 to make way for a lighter performance folding bike. Both the SL and TT are probably 2 of the best bikes Dahon ever made and both offer really nice rigidity and performance.
Now give me a Speed Pro TT with a titanium frame Drool~~~~~~~~~~~~~~
Last edited by mulleady; 09-06-08 at 03:23 AM.