Theoretical is the difference of the gear ratios ~ 11 %.
(Amuro has calculated that for us already here: http://www.stridaforum.com/forum/viewtopic.php?f=2&t=2688&start=0&st=0&sk=t&sd=a).
Practical is the felt difference not that much, because the 18" tyres (1,25" or 1,35") are not as wide as the 16" tyres (1,5") and the 18" tyres are normally used at higher pressure.
The 18" Kojak tyres do not have any profile, which is also reducing the roll resistance (disadvantage: less comfortable due high pressure).
For my feeling the higher gear ratio of 18" wheels is compensated partially by lower roll resistance of the tyres.
I’ve also tried 16" x 1,95" tyres (Schwalbe City Jet), these tyres needed a very similar, maybe equal, effort for pedaling like 18" x 1,35" Innovas.
Of course, the effect was different - the Innovas increased speed for ~ 4 km/h.
(The energy was “eaten up” by the higher roll resistance of the City Jets.)
As you own two Stridas now, it might be imaginable to use wide 16" tyres first for training purposes.
Or, change one bike to 18" with Kojak’s for use on flat roads and good weather,
the other 16" Strida could be used if you want to ride hilly roads, when it’s windy and raining.
Btw, Genuine_S wheels are of the same size like the usual Strida 18" wheels (possible tyre sizes for Strida: 32-355 and 35-355) 