Apple iPhone With Garmin StreetPilot Onboard App Vs. Garmin Nüvi 3597LMTHD

I currently have several Garmin GPS devices. The newest and most feature rich that I have is my Garmin Nüvi 3597LMTHD. I am pretty satisfied with its performance. I am considering getting another device for one of my vehicles and I am considering whether I should get another Garmin Nüvi 3597LMTHD or if I should get am Apple iPhone and install the Garmin StreetPilot Onboard App. I would likely get a RAM Mount and mount it on the windshield, air vent, or a CD mount.

What I am trying to determine is the plusses or minuses of each. Here is my short list so far:

Apple iPhone-

-More versatile. I could stream music through this to my automobile speakers.

-I could select Google Maps, Co Pilot, Tom Tom, or other maps as an alternative to Garmin.

-This would serve as a backup cell phone. I have Verizon on a Motorola Droid Turbo 2 currently. I would likely get some AT&T airtime for covearge when I am out of Verizon range.

Garmin Nüvi 3597LMTHD-

-Less expensive overall

-Since I already own one, the learning curve is a non-factor

Other factors that I am trying to determine are how well the traffic feature works on the Garmin StreetPilot Onboard App. I assume that it works just like dedicated GPS devices, but I also know what happens when I assume. With my 3597, I usually depend on the HD Radio traffic feature, unless I am out of coverage, and then I connect via Bluetooth to my smartphone and get traffic data that way.

Are map updates available at the same time for the StreetPilot Onboard App or are the available less frequently?

What about things like saving favorites and recent finds on the StreetPilot Onboard App? Do they pretty much behave just like the dedicated GPS devices?

I am somewhat interested in multi stop routing. It was a bigger issue for me when I was on the road all day, everyday for work. Now, it is pretty much a non-factor most of the time.