With a 1ZZ, the Euro 6 speed has a lower numeric final drive than the US 5 speed (3.94 Euro vs 4.31 5 speed) so the RPMS are lower at highway speeds and the top speed in each gear is higher. The SMT 6 speed is sometimes converted to manual and has the same first 5 gears and final drive as the stock 5 speed with an overdrive 6th, so the speeds in each gear are the same, but 6th drops RPM.
The 2ZZ seems to work best with the C60 as it has a higher numerical final drive at 4.529, so the RPMs are higher in each of the first 5 gears at any speed with an overdrive 6th. Since the 2ZZ powerband is at higher RPM than the 1ZZ, it is better matched to the C60
Examples of typical RPM at 70 in top gear (tire size dependent)
US 5 speed: 3,600
Euro 6 speed: 2,950
SMT 6 speed: 3,200
C60 6 speed: 3,400
Personally, with a 1ZZ, the Euro 6 speed is the nicest combination for more open roads and highways, but the converted SMT 6 speed is the nicest on tight roads with the same first 5 gears as stock and the overdrive 6th to drop the RPM on the highway. Most people with a 2ZZ use the C60.