You are correct. What you are currently doing wouldn't require more than 6 fast cores. I would say something along the lines of "the MP Xeons are a lot more expensive than their single counterparts", but then again over time getting dual hex-core processors will drop considerably (especially whenever Apple moves to Sandy or Ivy Bridge based Xeons). For what I do, higher processor counts are more important than single core speed (running multiple VM's as well as video encoding which is highly multi-threaded), so I would choose the 8-core over the quad core.
With that said, right now the single 3.33ghz hex-core processor runs about $600 new. Even dual hex 2.4ghz processors are running about $1200. It will still be awhile before the higher clocked dual hex processors come down in price (the dual hex 3.06 currently runs about $3,000 for the pair!). So if you are planning on upgrading soon, you would probably be better with the quad core because 1 - 3.33ghz Hex will crush the dual quad core 2.4ghz, but if you think you will be fine for the new 2-3 years (at least) before you upgrade then by then the dual hex cores should be more reasonable at that time.
It's a tough decision. Also depends on how long you plan to keep your Mac Pro. 2-3 years? Then Single Proc. 4+ then maybe the dual proc is the way to go.....