Ok John, my 2 cents worth.
I have an ST4S, and rode one of the first Multistrada's last year-in fact I helped 'run in' OnYerBikes demo
.
Now lots of the boys, Jools, Fordie, Keefyboy, were with me and the Multistrada rocks, VERY easy to turn chuck it in at stupid angles and just gas it out extremely confidence inspiring. My girlfriend Jo reckons it is more comfortable than the ST since it is longer legged for a pillion. But, the ST4S is much stronger at the top end so for long distance at 100-110 it wins easily, at that speed the 4's is just loping along whereas the Multistrada would be working HARD especially 2 up with luggage. The 4's is great in the hills but I reckon the Multistrada would be better. Sorry to not be much help but only you know how you ride so only you can decide-you've got to ride them mate.
I can only say that I won't be changing my ST4S even though it's nearly 3 years old with 20,000+ on it-but if I could afford a Multistrada as well........................
John