Iridium sat phone, which external antenna?

Hi, I have an iridium sat phone (9575) that has the litelock docking station in the fourby (mounted on the dash.) I am debating about which external antenna to get. There's the single whip antenna and the dual (phone and gps for $100+ more.)

Does anyone have the non-gps single antenna, and does the gps still work in the phone without the gps external antenna option in the car? Or does it really have to have the external gps antenna option to work?

I just figured the built in phone antenna should have the strength still for gps location data, while the external antenna helps with call reliability and clarity.

Thanks.