GTR has changed the game for high performance sportscar makers in many way IMO. For half the price of an exotic or less, you get equivalent performance (and better against some), a decent boot and 2 extra poodle seats. Only issue for most I feel is the branding (Japanese/Nissan after all) and the looks of it for some (subjective anyway). Maintenance wise is not bad...the M5 is a lot worse.
Really depends on what you are looking for. Pure performance? No question the GTR. Looks? subjective. Branding/Prestige/Conti feel...likely the maser. Exhaust note...maser but aftermkts on the GTR can narrow the gap somewhat but the GTR will never have the sweet V8 sound.
The price point you are looking at 300-400k puts you in play for masers, non-turbo porkers, V8 R8 and maybe some older AM V8 4.3s.
Personally, I'm also in a dilemma. I like those cars but none of them can match the performance of my GTR. Hence, I am more keen to look at bulls and horses (probably a horse for me) but they are obviously at a higher price point.
Good luck in your search! Both are great cars in their own ways.
Cheers.