Valid points here, thanks for commenting @adamdo! Agreed, if we're comparing strictly for astrophotography purposes, then both cameras are essentially equal :) I use a 5DMK3 sometimes for general work purposes, and it certainly is much more robust than the 6D, but the price increase was just a bit too much to stomach for a personal purchase. So I did exactly what you mentioned - get the 6D and some better lenses :P

One day I'll be able to afford the 5DMKIV...one day...