To say the same thing -- it really depends on the applications and the operating system you're doing, and the overall workload of the system. Almost nothing in Mac OS 9 was ever coded to take advantage of dual 604s. Among things that were, the list was very short and it was typically only specific tasks or sub-tasks within a larger workflow or application. For example, in Photoshop an SMP plugin could speed up some, but not many of the large application's functionality.
At almost everything you can do in system 9, a G3 will be faster. 233MHz is a good guess. If you could get any G3 with 1 meg of cache, that'll help a lot as well.
However, if you're running Linux or BSD on the system and you're running a heavily threaded workload, then you're probably going to see the system is overall "more responsive" with the dual processors, even if with a G3 (at, say, 233MHz) it could do single tasks faster.
One thing to note is that, say, for the 9600, there were 604e cards up to about 233MHz available, and of course you could get a Mach5/Kansas board and use a 300 or 350MHz CPU, which would (out of sheer cache) likely also outperform a 233MHz G3. So if this is about buying parts for, say, a 9600/200, then my recommendation is the faster 604 or a G3 upgrade, purely because that's what OS 9 and apps for it will be able to use.
Now, if you're talking about whole systems, rather than just CPU card swaps, then I would expect that the faster "literally everything in the whole computer" that came along with the Power Macintosh G3 (Beige and later Blue-and-White) would be noticeable, even possibly at lower overally frequencies, but I'd say that by 400 or 450MHz you're guaranteed an across the board performance improvement.
As it was alluded to in the other thread, this depends on you not needing more than 768MB (Beige G3) or 1GB (Power Macintosh G3) of physical memory for your task, because expandability (memory ceiling and slots) went down a lot from the 9600 tot he G3 family.
However if you needed 1.5 gigs of physical memory in 1996-1999 I question whether or not you were shopping for Macs, and maybe looking at a RISC/UNIX system instead. (This is especially true if your workload was UNIX to begin with.)