I have an e46 M3 and i have to say that it's an amazing car, I have basic mods: coilovers, swaybars, intake, axle back.
I've been considering getting something a little newer and the FR-S has really caught my eye as a cheap no non-sense sports car
The M3 is a fantastic car, not the fastest in a world of 420hp hp mustangs but definitely still fast enough to get you in trouble.
The biggest thing everyone always says is that maintenance will kill you, and that's a half truth. There are some very expensive services that need to be done, but not that often. It works like oil change -> oil chance -> major service. The major service can run you anywhere from $1500-3000 depending on where you go however it's quite extensive and requires that they keep the car for the day or even overnight as they're changing most of the fluids in the car. The oil is also very very expensive, but i only change it once a year (more often than the car actually requires) so it's not the end of the world.
The big problem is when something breaks (especially anything engine or transmission related) out of warranty and you're looking at serious dollars. You should have a good mechanic that you trust,Ii do and luckily i've been spared any major repairs since i've owned the car (only an alternator so far). Luckily it is by all accounts a relatively bulletproof car if you keep up the maintenance.
How does this car stack up to the very brief autocross drive of an FR-S I've had?
They're completely different. The M3 is way heavier, although much more powerful and easier to take sideways. Turn in on the FR-S is crisper and more urgent (despite the suspension work on the M3), the transitions in the FR-S were faster and easier to make, and overall the car just felt lighter on it's feet.
The M3 is still an amazing driver's car though, on some twisty back roads with the engine singing above 6000rpm is the closest i've been to driving nirvana in my admittedly short life.
The interior on the FR-S leaves a lot to be desired compared to the M3 unfortunately, despite being much older my M3 has options you still cant even get on an FR-S like backup sensors, memory seats, motion activated alarms, 10 way power heated seats, etc. You are definitely losing out on some luxury with the FR-S. The M3 also has a useable back seat, I'm 6' 180lbs and my 6' 225lbs friend can sit behind me and has enough leg and head room to be comfortable, that will never happen in an FR-S.
My M3, 2004 alpine white 6 speed manual:
