I've been living in Suzhou, never been to Xiamen. From what I can read about Xiamen
* Suzhou have 2X more people
* Suzhou's winter are way colder (5c to -5c for 2 months, 0c to 10c for 2 more months)
* Suzhou is not next to the sea, you have lakes, and you don't want to swim in those lakes
* Both cities are amongst the most liveable in China
* Suzhou's cultural life is vibrant... by Chinese standard, which are very low. I've seen 50 000 people towns more active on that department.

Xiamen would win for me, because of the climate and the lower population ^^ But you would visit both cities for a week-end.
 

I'll answer this one for the fun of it and the extra points. I heard Xiamen has nice beaches.  I admit, it would be a tough decision.  On the other hand I met a good looking young hooker from Suzhou.  So I choose Suzhou.

I live in Suzhou and think it is a great place:

25 minutes to Shanghai by train

2 subway lines

Beautiful gardens and parks

Plenty of western conveniences

Lovely ladies

Suzhou does it for me, I've been here near 7 years, it just so beautiful. Near also to beautiful Zhangjiagang which would be my 2nd choice. However I guess it all depends just what you are looking for. Long term, short term, work, play or just relaxing life style.

Good luck with your quest.