formulaben, I am not talking about INS. The PL21 will take whatever inputs you provide it, including INS. The Garmin is a GPS, period.
Flying around non-oceanic, if you lose the GPS on the PL21, you go to Rho Theta without a hiccup and continue RNAV. On the Garmin, you lose RNAV and are back to VOR to VOR.