Not quite your situation - but for what it's worth ..
I have a 2008 2.8 Ghz 8 core MacPro w/16GB ram and a 2012 mac mini server 2.0 Ghz i7 quad core processor w/ 8 GB ram I use a part of a render farm.
The 2008 8 core produces files faster when running compressor 3.5 and Qmaster.
Other things to consider -
What video card do they come with?
Can you add a second processor to the 2009 machine?
How much RAM do they come with?
All things being equal, I think I'd go for the newer machine. Computers do wear out/burn out/fade away ...
And the winner is...the 2009 Quad Core Nehalem. Even though the older machine benchmarks somewhat faster than the 2009, I decided to go with the newer technology. I am trading some rendering speed for a machine that will be useable a few more years if Apple makes another major change to the OS. The video card it comes with is OK, but it'll be getting a new ATI Radeon HD 5770 and 16GB RAM.