My previous car was a Euro 321bhp M Coupe, slighly different from the 240bhp ones in the US. Until the S54 engine came along that is

Mine has the S50 engine. I had it for 3.5 years and was my only daily transport. I'm now driving a Boxster S.
In terms of power, it is just awesome. Of course, I had a couple of usual things done to it like SuperSprint exhaust, Gruppe M carbon intake kit, UUC short shifter, adjustable camber/caster kit, etc. The sound of the engine above 5,000rpm is just too awesome to describe. The power is stunning on that car in a straight line. I've outrun a stock Evo on the track straights and heavily modified Nissan S15s. The gearshift is short, precise and sweet contrary to what some mentioned here. Fuel consumption is very good for such a powerful engine... bet 7.5-8.5km/l
The brakes on the M Coupe sucks big time. Double sliding pistons up front and single sliding pistons in the rear. Fades big time on the track even with my upgraded Tarox floating rotors.
The downside is really the trailing arm rear suspension that was inherited from the E30. This suspension is very prone to camber changes during suspension travel. Hence, the reason for sudden snap oversteer during corners. It did scare the daylights out of me a couple of times on the track..... and thank goodness it only happened on the track for me. A few M Coupes/Roadsters have been totalled this way on public roads, especially on roundabouts in UK.
Interior wise, I think it's comparable to the Boxster. But my Boxster S came with full carbon interior which I thought looked much better than the M Coupe's. It has additional things like oil temp gauge that is lacking on the Boxster.
After owning my Boxster for 6 months now, I think the Boxster wins hands down for steering feel, brakes and suspension. The M coupe wins in terms of straight line power. That power is useless in corners because of the suspension. Power out too early and you'll be looking the other way on the track at on-coming traffic.
The power in the Boxster is still lacking in my opinion. You just can't beat the rush of straight line acceleration of the M Coupe or the Evolution (my car before the M Coupe). And the tiptronic doesn't help
Hope this has been helpful
One last note, the body of the M Coupe, though supposedly much more rigid than the M Roadster is still less rigid than the Boxster. Lots of creaks and groans as you feel the chassis flexing.