Question on Unlocking iPhone 5 / Changing Carriers

I am seriously considering paying $300 to end my contract with AT&T, unlocking my iPhone 5 and going to T-Mobile. I currently pay around $190 for two phones, iPhone 5 & a generic flip-phone on AT&T and I've had enough. I cant justify the cost. I'd like to know if anyone has any insight on the process, do I loose my mobile telephone number with the switch? Will I still be able to sync my iPhone with my Macbook/iTunes? How is Chicago's T-Mobile network?

Thank You in advance!