Either seems to be stiff enough for me.
I found a clean coupe and V8 swapped it for a track-only car, and it's fine.
We even had too many guys working on it at the same time one day and dropped it off the support blocks, resulting in them making pretty sizable crimps in the floor pan on both sides directly in front of the rear wheels. That was a very bad moment in time.
This happened early on, and it kind of stunk doing all the work to finish the V8 conversion, all the time wondering whether I would need to start over with a new chassis once I got it rolling and it turned out it was crippled. But, no alignment problems resulted and the car is fine at the track.
If I was going to haul extra wheels to the track in the car itself, I would prefer to have the sedan. I have to load race wheels into the back seat of my E92 M3 coupe, and it is a bit of a hassle. Once I cage the E36 coupe, the idea of getting anything into the rear seat bench through the doors will be 100% impossible.
The coupe I bought had fold-down rear seats. I don't know if that's standard. But I was able to strip the seat backs down to the metal frame panels, and they clip right back in or out easily. That gives me a bit of cargo flexibility. I don't use a fuel cell, so I have been assuming it's not risky to run without an airtight firewall to the trunk. Maybe that's unsafe?
There is one thing I didn't factor in when I bought a 325is for a V8 swap. I was hoping the brakes would be adequate for the track, and I could keep running the 15" wheels for tire (and wheel) economy. For me, the brakes weren't close enough to good. That meant I needed to do the E46 330i/325i caliper/rotor/bracket conversion (which worked great for me) and source three sets of 17" wheels.
Also, the 325is I got had an open diff, which was not close enough to good for track work. So, I didn't pay a premium for an M3 chassis, but I did need to pay extra for all those parts.
And I think the M3 chassis might come from the factory with the rear subframe reinforcement plates. I had to install those myself. I don't like welding on the underside of cars because I'm north of 50 and don't have a lift. But, it's not that hard, just incredibly tedious.
The damn thing was useless with the stock springs and the OEM rubber bushings. And the bushings aren't easy to change. If I stumbled on a rust-free chassis that had the rear subframe and Rtab reinforcements and bushings already done in urethane, I would pay up $500 just for that.
The coupe fenders were really, really easy for me to roll.
Getting adjustable rebound dampers and height-adjustable springs on all four corners let me balance the suspension to a much better place. I have a ton less understeer and more stability in the brake zone. I didn't need camber plates. Just getting the M3 front strut mounts and swapping them side to side was enough.
