really depends what you want. smc is 1900 sm4 is 2350. worth the extra money i think. i personaly have a smc. and have had for well.. about 5 years now. it hasnt missed a beat. its been rained on,welded unprotected,had smoke come out of it(my fault) and still going strong. so robust its just not funny.
ony drawback on smc is limited options for crank triggering and axillary outputs.you also have to choose hi or low curent injector drivers. i have the autotune feature and a wideband. but i leave that to my tuner. it takes around 30 mins to map the fuel from scratch and its dead acurate. futher to its reliability is being able to clock up 240000km (120k over 300rwkw) on a standard r32 gtr enging. i ended up changing it just cos i was bored with it.. no detonation at all.
summing up its a very good ecu, very easy to use and tune. so for a tune dont expect more than $450 not 1k on a pos power fc with airflow meters that f**k out etc etc.