Having owned a BOSS 335 and now the FN Mustang, I'd have to pick the 'Stang because it does everything just a little better than the Falcon and that's simply because they are no longer being made.
My heart is still in the FPV but there is no denying the two cars have always had a close association with each other from the very beginning... eg. Ford Australia marketed the new XR Falcon's as "Mustang Bred" and you can clearly see the classic Mustang lines in the side profile.
Fast forward to today and since 2010, every V8 powered FPV has had a Mustang block in them. Yes, there is nothing like Aussie muscle but let's not forget that while Moffat helped seal the legendary status of the '71 GT-HO Phase III's at Bathurst, he also piloted a '69 BOSS 302 and won 2 out of every 3 races in it with the angry 5.0 beating cars with big block engines.
BOSS 335 vs GEN3 Coyote...
The Miami runs 9.2:1 compression and 8psi of peak boost to make 500hp @ 5900rpm whereas the Coyote has 12:1 compression and makes around 480hp @ 7000rpm from the factory. The Miami obviously has more torque but the Mustang's 10-speed auto makes up for this... just to show you how short the 10R80 gears are.. where the 6-speed ZF in the FPV can reach 165kph at the top of 3rd, the Mustang does this at top of 4th.
Handling...
The Mustang is just a level above the FPV here with a better sorted chassis and suspension (especially with Magneride) and stickier Michelin tyres but to be fair to the FPV, it is a 4-door sedan carrying an extra 120kg with a higher centre of gravity. The FPV's have better steering feel than the Mustang's electric setup though
Cabin space...
The FPV wins this especially when it comes to carrying your mates in the back seats. The Mustang's rear seats are only good for people under 5' 8" but the boot space is comparable with 408 litres vs the Falcon's 535 litres. Front seat space in the Mustang is decent for a sports car with lots of headroom.
Interior Tech...
From Sync 3, digital dash, radar cruise control to the indicators sounding like a galloping horse, the Mustang wins this round. Loading my entire music library onto a tiny USB drive beats having to use a 16GB iPod and having to pick and choose what I can listen to in the FPV.
Exhaust noise...
This will be subjective but stock for stock, the FN Mustang is so well sorted from the factory in track mode, that I'm actually leaving mine as is whereas 2 weeks after owning my GT, I had disconnected the bi-modal and replaced the centre muffler with a 4.5" resonator. The reason the GEN3 Coyote sounds so good is partly due to the high compression and 7400rpm redline.
You can listen to mine here..
https://www.youtube.com/watch?v=VXvH5dAiNfM
Another plus for the Mustang is the fuel consumption... 10.9 vs 13.5L/100km for the FPV driving in the same conditions.. so even with a 10% smaller tank, the Mustang has more range (560km vs 504km) than the Falcon.
One last thing... they are both GT's and they are both Ford's !