SUzhou. No question, at least not IMHO.
There are really 2 Jiangsu Provinces...North and South.
Xuzhou is North Jiangsu, Suzhou is South Jiangsu.
South Jiangsu is rich, educated, pretty developed, faster-paced, has lots of foreigners and foreign goodies, and based on high-tech industry and tourism.
North Jiangsu is poor, less educated, much less developed, bucolic, will take a 2-hour bus ride to buy cheese, and is based on smokestack industries and agriculture.
South Jiangsu is culturally tied with Shanghai and with northern Zhejiang province as far as Hangzhou (taken together possibly the richest and most advanced area in China).
North Jiangsu is culturally tied with Anhui province and eastern Hunan province (taken together perhaps the poorest, most backward, most staring-intesive area in eastern China).
Much depends on you and the kind of experience you want, of course, but personally I wouldn't take anything much north of Nanjing, the city where the 2 Jiangsus meet. The South is just too much more interesting, comfortable to live in, and full of opportunity.
My opinion...