Concillian: When the OP asks if it is worth it, I usually never answer yes or no. $50 to one person has more value than $50 to another. I only like to provide information so that him/her can answer that question on their own terms. What the OP may also be overlooking is the cost of the motherboard in the equation too. From what I've been looking at lately using the 250 chipset as a guide, a 754 mobo can be picked up for as low as $75, where the minimum 939 mobo is costing around $115. So now the effective cost of that processor is $90 additonal.
The points you make are valid as the comparisons are not exactly apples to apples. But in the broader scope, I don't see 9b vs. 9c making too much of a difference.
Here is a 9b to 9c comparison on synthetic benchies and
here is some SP2/9c vs SP1/9b info on OpenGL. If anything, the OpenGL benchies imply the 939 scores may be slightly lower if run on SP1, making them closer to the 754 scores.
In addition, and at least in my opinion, the existence of 754 is for the lower end market at this point, ie value for $$$. So if I am looking at the entire package vs. 939, I'm looking at low cost mobo, cpu, and ddr. Like you stated, it is nearly apples to apples at 2.0ghz, so the difference between platforms can be seen there. At 2.5, the 754 needs to have the ddr backed down, as may very well be the case in a "value" system. IMO this gives a decent idea of what your giving up with lower cost ram that may not do 250, but will do 216. And you can see that the performance hit is not that devastating to yet spend even more dollars on ddr in a "value' system.
Maybe I should have expanded more in my post, but the points that you thought were not a good comparision were what I liked the most.