Whichever method you use, be *sure* you get a WAAS GPS, else you'll need to completely ignore any altitude readings coming from the GPS. Even WAAS isn't dead on but it's a heck of a lot better than +/- 150 feet altitude reading. Air pressures (temp compensated) are way more accurate than GPS for altitude.
I use stratux as ADS-B in with a WAAS GPS and looking for a way to add the air presure readings.ao I have both (besides the steam gauges on the panel.)