Hi,
Real-life MacPro benchmarks are very hard to come by, so it's always hard to determine how much we'll gain with a given Mac. There are artificial benchmarks, though, and Primatelabs compiles all Geekbench results into one table, found here:
http://www.primatelabs.ca/geekbench/mac-benchmarks/
The two computers you're looking at are the "Intel Xeon W3680 3.33 GHz (6 cores)" and "Intel Xeon E5620 2.4 GHz (8 cores)", which score 13904 and 12525, respectively. A 1000 score in Geekbench is the equivalent of a 2004 1.6gHz G5, so the 6-core is about "one G5 faster" than the dual-4-core. This means very little in practice, especially in PT, but gives a relative idea of maximum performance.
Going from a G5 to any of these two machines will be scary-fast, the 6-core being a tad scarier-fast. If that is worth the extra 200$ is down to you, really.