Unlike everyone else, I'll actually give a reason behind my answer.
The memory bus on the G4s is crippled. You won't even get the listed speed out of it. The faster memory bus on the G5s is not crippled. You will get the listed speed out of it.
Using software that can take advantage of multiple processors, a dual 1.42 (or even 1.25) will be faster than a single 1.6. But for single processor tasks the 1.6 will be faster.