Looking for some advice on buying a SIM card for use primarily in Shanghai. I fly into PEK first, stay 1 day, then fly into SHA, stay 1 week. A few moths later, I have another trip going back to Shanghai staying 3 weeks. I understand that China Unicom prepaid SIM is probably the best for me for a mix of voice, SMS, and data.

I heard it is easy to buy a SIM card in PEK, so should I just buy it there first before going onto SHA? Or is it going to cost more to buy at PEK and use in Shanghai? Is it easy to buy a SIM at SHA airport also?

Thanks in advance for any advice.