I have had first hand experience with both of these drives, so I'll throw in my 2 cents.
The 60GXP wins in seeks, while the Cuda IV will have a higher STR(40GB aureal density vs. 20GB for the 60GXP). That's Sustained Transfer Rate Btw.
The Cuda IV is definitely the most quiet of the drives. While the 60GXP is definitely a very very quiet IDE drive, it can still be heard if you have a very quiet computer and are close to the case. I can guarantee you will never hear the Cuda IV once during its whole lifetime.
So there you go, you deal with lots of small files, the 60GXP may be faster for you. If you load lots of big programs though, the higher STR of the Cuda will probably let it pull ahead of the 60GXP in perceived "quickness". If you want an absolutely silent drive, go for the 'Cuda IV.
BTW, both are great drives, and my 40GB 60GXP has lived through hell with me for the past 6 months(i.e. routinely moving, copying, deleting 1GB+ files and being on 24/7 since it first powered up).