Old GPS or Ipad Mini for boat navigation

Depends on the iPad. Some have GPS capability and some don't. I used to run with an iPad with a downloaded coastal GPS app up and down the coast, but the coast line on the app was outdated. Worked well enough, though, because of the GPS receiver.

Ran a tab as a chartplotter for a while. Very hard to see in the sun.

my experience has been that the marine environment is just harsh as hell and even instruments designed for that sometimes don't hold up well...I would check to see if there is some software update that you could do to make the garmin useable, as I would be scared that the ipad won't last long...

actually, since navigation is so critical, I may take them both if I were you....