Re: 156 2.0 twin spark Tyres and Alloy sizes???
16" seem to fit the 156 quite well but make sure your certain as in 6 months time you don't want to wish you had the 17".
wheel and tyre deals are unlikely to be much different either 16" or 17" it all depends on who has just bulk ordered what from the tyre manufacturers.
pick 16" by all means but check the price first if thats you only real issue. although judging by your choice of wheel your not looking to scrape the barrell.
its worth finding a stockist with 16" & 17" in stock as you can assess them against the car to see which looks right. pcd is unimportant just get the greasy chap in the overalls to hold it over your current wheels and have a good long look. the overall rolling radius will be the same so you can see the effect of a lower profile and what looks right in the arches. a side effect on some cars having larger alloys is that it looks like the car is on stilts but 16" or 17" should n't affect this too much probably only 18"+
If ignorance is bliss, why arenít more people happy?