They may be similar in price, but definitely NOT similar in performance, reliability, design, or aesthetics. A more realistic comparison with the SC400 is the G35 coupe or the IPL G coupe (and the SC fails miserably when compared to either of those). The most obvious difference is of course that the M45 has two more doors, is quite a bit larger and heavier, and the SC is less powerful, has fewer torques, is slower from 0-60, and generally, is not a looker.
The only area where I will give the SC400 a leg-up is in fuel economy. And at best, it gets maybe 1-2 extra mpg over the M45.
Now, if you meant the LS400 sedan, that's a different pickle altogether! However, the LS430 is a more logical comparison. The LS had a 6 speed gearbox to Infiniti's 5 speed, but much less BHP and torques than the M45 (LS: 278 Vs M45: 340). That said, the M45 was built as a luxo-sport sedan while the LS 400/430/460 are built for pensioners who's main priority is comfort, quiet, and luxury. The interior of the L-class Lexus puts Infiniti well behind but the L class Lexus feels like ya grandad's sofa and if his sofa did have wheels, the performance would be similar. The M45 is more agile and sporty but also less cushy and more noisy than an LS. When it comes to repairs, both are very reliable cars but the Lexus LS is known for extremely costly bills when the rare failure occurs. Infiniti is definitely no bargain, but in general, cost slightly less for the same repairs.
I would sum it up by saying, if you want a smooth, quiet luxurious ride, you can't beat the LS 400/430/460. If you want a comfy, but more nimble and powerful sedan, the only choice is Infiniti. Zee Germans make only stiff riding sedans until you reach the 750, A8, or S class models and by then, you're well above the M45 and Lexus prices for ownership and repairs. And the Americans have given up sedans completely, having consistently failed at making a luxury OR sport sedan, much less a sedan that can do both and still be reliable and last more than 5 years without falling apart.