I agree...no real need for gps on SXM.

Phone: for use inside/outside of rental= Sharon (reasonable cost)
Own Phone: for use on wifi (wherever)..Skype (airplane mode enabled) (cost minimal)
Own Phone and data(assuming you have an unlocked phone) sim card (cost depending on data used)

AT&T(like Verizon) has a $40/month plan also but adds $1/minute voice and also a limited data allowance. Probably not appropriate