3dMark scores largely depend on memory and graphics card horsepower.
Were the memory modules in these two comps identical? What were the timings? Was a divider used?
What kinda vid card was in each machine?
Remember that 3DMark is just a benchmarking tool. I would be more concerned with real-world performance weighing in on my decision.
As far as which is a better chip to overclock, it all depends on what you want to do and how high you want to get. Unfortunately, there's no way someone can give you a straight answer. However, I will brainstorm a few things to consider:
First, 2.4C's tend to run cooler than 2.8C's, so if you're not into watercooling, that might be one advantage to go w/ a 2.4C, as you don't have to bend over backwards with cooling strategies to get a decent overclock. Also, ZipZoomFly and Azzo are still selling m0 stepping 2.4C's, so there's a good chance you'd get a good one if you ordered now/soon.
On a decent 2.8C with good air cooling, you could probably get a 3.5GHz o/c.
On a decent 2.4C m0 with good air cooling, you could probably get a 3.5GHz o/c.
Another thing to consider is that if you get a 2.8C, you'll probably need to get better memory (PC3500 or better, as PC3200 modules would limit your o/c) for a good o/c than what you'd need to get if you got a 2.4C (they do well with PC3200 and up).
Basically, it all depends on your own goals, cashflow and what components you use to put it all together.
Goliath