En estética de motor me gusta más el tuyo Mig.
El culo de ambos (S50 y S54) es el mismo? o el S54 lleva los faros post?
De "chapa" son iguales.
Cambian pequeños detalles como los logos (planos en el S50 y con relieve en el S54), colores, y cuatro pijadas más para los más elitistas.
A nivel técnico, cambia motor, se añade el DSC, el autoblocante no es mecánico ...... y poco más.
Son muy iguales.
La mayor diferencia, es que del Z3 M Roadster con el S54 no se fabricaron ni 300 coches (para Europa). De ahí lo exclusivo 
lo que más cambian es el color de las llantas en mi opinión, depende del color pero me gustan más las del 321
What are the differences between the S50 and S54 engines?
The S54 powerplant of the later M roadsters is technically an evolution of the iron-block S50 B32 unit used in all European-spec M roadsters built through June of 2000. Although the peak power and torque of the S54 (325 hp at 7,400 rpm and 261 lb/ft of torque at 4,900 rpm) are barely increased compared to the S50 B32 (321 hp at 7,400 rpm and 258 lb/ft of torque at 3,250 rpm), they share few major components and differ in many areas including:
-Increased cylinder bore to 87mm (from 86.1mm) for a new total displacement of 3,246cc (from 3,201cc)
-Modified camshafts
-High pressure Double VANOS continuously variable valve timing system with faster operation at high rpm
-Increased compression to 11.5:1 (from 11.3:1)
-More advanced BMW/Siemens MSS 54 engine management control
-Finger-type rocker arms for reduced reciprocating mass and friction
-One-piece aluminum head casting for lighter weight
-Scavenging oil pump to maintain pressure during heavy cornering
The S50 B32 unit can be is identified by the "BMW M Power" inscription on its cam cover, while the S54 contains only the "M" badge