I found myself in the same situation as you a year ago,when we finished my Garmin panel. I was a ForeFlight user for years before the Garmin panel, so Garmin Pilot was new to me.
I purposely decided to give Garmin Pilot a shot. After using both apps for about a year, I find things that I like about both. I would say both ForeFlight and Garmin Pilot generally do the same things. You can plan and build a flight plan, graphically edit, or type in, etc. Some of the things that I like about Garmin Pilot is that it is the same user interface as the GTN navigators, and the new TXi flight decks. I like how Garmin Pilot integrates weight and balance and fuel loads with your flight plan. I also like how Garmin Pilot has quick fuel loads in the Weight and Balance section. If you load up your plane with people, you can quickly select max fuel and it will give you the maximum amount of fuel without going over max weight. ForeFlight makes you figure it out yourself and manually put your fuel loads in.
Garmin Pilot has a dedicated traffic page with the black background and concentric range rings, ForeFlight does not.
I like how ForeFlight has audible call outs for ATIS or AWOS etc as you approach your destination airport. The runway incursion call outs, etc. I like how you can put N numbers into ForeFlight and get aircraft info. I like ForeFlight’s METAR and TAF layout better than Garmin Pilot. I also like that ForeFlight provides MOS weather forecasts, Garmin Pilot does not.
ForeFlight lets you use its vector based aeronautical maps over top of a VFR sectional. Garmin Pilot does not, it’s either or.
These are just a couple quick examples of a few things that one does better than the other, certainly not an exhaustive review.
For me, I find myself going back to ForeFlight I think in part because of my years of working with it, I am still more comfortable with it. I still give an edge to ForeFlight personally.
That being said, if I was forced to give up ForeFlight and only use Garmin Pilot, I could get along fine with that.
Both programs will receive traffic and weather, and flight plan transfer, but ONLY Garmin Pilot will do the the WIFI database transfer, so you will at least need to have Garmin Pilot for that.
One thing to be aware of: You get a free year subscription for Garmin Pilot when you buy a Flightstream 510. So you have a year to use it and decide for yourself what you think of it.
On the other end of the spectrum: My Dad exclusively uses ForeFlight for planning and in cockpit use. He has zero interest to learn another flight app. So he has Garmin Pilot solely for database concierge. He does not even update the Garmin Pilot app data other than to download databases to transfer to the GTN 750.
So for him it is just a $75 yearly fee in order to use database concierge. View it as a yearly fee for the convenience of WIFI database transfer. It is much more convenient than pulling SD cards and updating on a PC. Once you use it, going back to pulling cards is just clunky.