Interesting. I imagine it's a bit like the difference I experienced between a 500 with a 1.2 vs. the 1.4 - the 1.2 felt nippy around town, even a little more than the 1.4 thanks to carefully-chosen gear ratios, but it ran out of puff at higher speeds and getting up hills (where the 1.4 had no problem).
You might find that cruising at a sustained 70mph on the motorway (particularly up and down slight hills), the 1.6 may use a bit less fuel than the 1.2 simply because it's not having to work as hard, regardless of the 5th being a little lower than 6th of the 1.2. I had similar experience with other cars, though not as heavy as the Stilo (1.2 is a surprisingly small engine for a car that size, which is why I suspect it would have to work quite hard).
I suppose the reason for my interest in this is because the Stilo was only sold in New Zealand with a choice of 1.8 or 2.4 petrol engines - no 1.2 or 1.4, no 1.6, and no Diesel engines... and no Multiwagon... so it's interesting to learn about what we missed out on.
-Alex