An E5520 will run at 2400 MHz when loaded compared to the W3690 which should have no problem running at 4000 MHz. The W3690 is better binned compared to the W3680 so it might be able to run reliably at 4133 MHz.
2400 MHz X 8 cores = 19200
4000 MHz X 6 cores = 24000
That means in multi threaded the W3690 will win and in single threaded, it will be almost 60% faster.
The E5520 are 45nm Nehalem CPUs. Two of those will suck lots of power, create lots of heat and the fans necessary to keep them cool will drive you crazy.
A T3500 with an overclocked 32nm Westmere W3690 is almost silent in comparison. It will run cooler and consume way less power.
Not really. W3690 still wins.
This really isn't a fair comparison because you're overclocking a cpu that can't be overclocked on the z600, so you need to compare based on stock speeds. For argument's sake, we'll use the base clocks without turbo:
2300 MHz X 8 cores = 18400
3500 MHz X 6 cores = 21000
With the e5520, the w3690 will provide slightly better performance. But w3690 is the very top end processor, which will cost more as well (currently $70+ on ebay).
And the e5520 can easily be upgraded to something like the x5670, which will be pretty cheap as these are just 'mid-line' server processors (currently $30 for a pair on ebay). Then it looks a lot different:
2900 MHz X 12 cores = 34800
3500 MHz X 6 cores = 21000
Here a dual x5670 z600 absolutely kills a w3690 t3500 in overall processing power.
And again I'd pick the z600 because of the dual processors as well as the ability to use at least 16GB ecc reg modules and possibly even 24GB and 32GB modules (I haven't tested them in mine yet) for 96GB, 144GB and potentially even 192GB of ram (I have 96GB in mine).