I don't quite see how the two compete, very different cars and both great but serve different roles and purposes. For me the Mustang was never really considered, too big, more a cruiser, possible more thirsty (minor point). Only sat in one, nice inside, looks great, sounds great.
For me the RS brand has been a long term affair, had 3 previously so I bought the Focus.....
To pick up a few points and internet hearsay......
Mk2 vs Mk3 the Mk3 is all round better car, it will also obtain the same "status" over time... Mk3 has had more praise and hype lauded on it the Mk2 did.... Okay the Mk2 made a quirky noise thanks to it's Volvo engine (So did my V40-D4 LOL)
Noise ... Sadly the FRS uses fake sound through the speakers... it's fun for about a day, then you just want to turn it off.... but Ford don't give you an option..... but searching the internet does. So now I have a little switch in the glove box and I can turn it on and off.....

It's natural exhaust noise is quite good.... (obviously you can buy other systems). Sounds good to me and people who have been in it.
Seat height..... I have the fixed height shell seats.... it even puzzles me why they didn't put a height adjust on them.... and I'm an automotive seating and interior engineer!!!! That said you get use to it very quickly, the hype is bigger than the issue. (BTW my other car is a Z4 ... not an SUV)
Have the RS updated with the Mountune FPM375 kit (£1099 and keeps Ford warranty) it's how it should have come from the factory... "Now it's right proper quick" .... increase in mid power and torque is great, much more urgent and alive..... nice little induction roar and turbo spool up noise as well.
There's an audio bug, software update greatly improves it and makes the rear speakers work.
It drives and feels very RWD, not like all the other FWD hot cars.... But you need to "Re Learn" .... Use the power to steer it and tighten a line into a corner.... awesome..... Made all the better by the Mountune 375 remap.
Anyone who thinks it's not nimble hasn't driven one, it's change of direction and turn is unreal... to the point where it could catch people out till they have adapted to it.
Interior.... Oh people love to slate a Ford..... Well the latest Focus interior is an improvement on previous. The difference to other brands only comes down to how much they spend on using more soft touch areas. They are all made from the same plastic. Ford chooses to spend it's money on Chassis dynamics, VAG group on more soft touch wrapped trim. All down to brand image and budgets.
I think you look at both and one will speak to your more than the other....either way choose Magnetic grey

Hopefully Ford keeps this going and we get more great cars.