I would imagine that at the level of Davis/Purdue, the choice will come down to a few faculty of interest, and their personal interest in taking you on. Also, as others have said, the culture of the two places are very different, and might play a significant role in your eventual choice.
Rankings are relatively meaningless.
Browse the faculty pages of both schools and see which looks more interesting to you.
Remember, it's going to be ~6 of your life. You should probably make sure you'll enjoy it. High ranking from USNews or NRC or whatever means nothing if you hate your life for 6 years. Another thing to consider is would you rather live in Davis or Lafayette?
Also weigh in how much they are paying you and the costs of living of those places.
Thanks for the answers.
Then can you(or the other guys) tell me more about how different culture between UCD and Purdue?
If the subtle ranking difference is not critical issue, the atmosphere or culture may seem important factors for me but I have no idea at all.
As imagination from web searching, I prefer more sunny climate in California as well as proximity to Sanfrancisco and Secramento than those things in Lafayette. However I don't have any real expriences in both area. Youguys, natives, would better to tell me about that.
As to funding, as you know well, I got tuition waivers with TA stipends from both.
Monthly stipends in both are not much different but Purdue said that they can't assure summer(3 months) TA jobs although they would try to give a TA support. Also stipend for 9 months could be reduced by 1000$ if I could not pass the English proficiency exam for international.
UC Davis assures summer TA support(around 6,239 $) by refering on an admission letter. UCD also said that they would not cut down stipend or fire me in the TA job even if I could not pass the Elglish proficiency exam. It makes me feel safer.
About faculty, by website information, Purde have a litte more diverse and number of faculty in CME than those in UCD.
I did not decide more detail major in CME yet. It confuses me.
However, do the safer financial support and good feeling(?) in UCD have some bigger benefits relative to Purdue's things, in veiw of opportunity costs?
Emotionally I am inclining to UCD, but rationally to Purdue. It seems tricky dilemma for me.
Welcome for any comments.