smaller wheels will be cheaper on rubber, and 16" alloys about 100 quid cheaper than 17 at halfrauds as well,
but if the track bug bites you may be looking for bigger brakes in which case the most powerfull ones tend to fit 17" although you can go 16" with
jcw and wilwood and there are some effective oem size pads and discs available all becomes a balance between track and road and also how deep are your pockets