Before you invest money in this enterprise....give it a whirl for awhile and see if its worthwhile. You may find that it is not all that it is supposed to be and quit shortly thereafter....of course if you just want a table and need an excuse, that's another story
Google map works great on your phone especially if its an iPhone 6. But under tunnels can really suck if you don't know your city's streets that well. Between large buildings can be a little tricky too especially if you are not good with directions, but you'll get used to it and save a bunch of money by not buying unnecessary crap for this minimum wage gig we call ridesharing
I have a 7" Garmin that I use. Last night was my first night (I already had the Garmin) & I can tell it made my night easier. It takes a few more seconds at the beginning of the trip but the bigger screen & better directions helped a lot.
I have a Garmin as well but I find that it is too inconvenient to put an address in every time. I do keep it on map view all the time and will reference it if I miss a turn or need help getting out of the neighborhood after drop off