the xeon will not feel faster or benchmark faster unless you have a very memory-bandwidth specific scenario where the wider memory bus of X58 (compared to P55) will come through.
other than that, they are practically identical but architecturally the 860 has a new take on power management, they threw away the QPI hub give you fewer PCIe lanes on the north side. as a result of this the i7 860 and W3550 work on different sockets and are not interchangeable. The W3550 is more a sibling of the i7 920 than the 860.
X58 on the other hand gives you three memory channels instead of two, and a whole lot more PCI express connectivity. This is the preferred workstation platform because the idea is that you're running a lot of high performance peripheral devices and you need a way to connect.
the xeons support ECC but it isn't necessary for more "accurate" calculations in typical computing as the error rate is small enough that you would need a very large and time consuming workload for bit cell errors to corrupt your work. if you aren't running a server or doing any 72-hour render jobs, you really don't need ECC because it slows you down by a couple percent.
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