Harman Kardon HKTS 18 vs HKTS 14

[Grimfate126]

Hi. Ive been hunting for a good speaker system for less than 500, and zoomed in on the HKTS 18. However, i just realizer that the HKTS 14 has a 2" bigger subwoofer (same wattage, 200), and is cheaper. The 18 has heavier tweeters. Other specs are identical. i will be using the onkyo TX-SR605 receiver.

So, my questions are

1. how big of a difference will 2" make (for the subwoofer driver)? my room is about 17'x14'?
2. does anyone have another suggestions for speakers (less or equal to $500)?
3. Do the heavier tweeters mean anything? or is it just the casing material?

Thanks.