JamesUCF :
I would recommend the M600 Processor for one major reason. The T6600 does not support Virtualization. This means that if you upgrade Windows 7 to Professional or Ultimate you can not run XP Mode or create VHD. These are advanced features but for the same cost I would want the feature. In my case I bought a Laptop with the T6600 and realized this after upgrading. I sold the T6600 Laptop to a friend and bought the M600. There are other Intel processors that don't support Virtualization so check the Intel site before you purchase.
From AMD.com "All AMD Athlon™, AMD Phenom™, AMD Phenom II, AMD Turion™ and AMD Opteron™ processors shipped by AMD will be able to use the AMD-V™ feature, and subsequently benefit from Windows® XP Mode in Windows® 7"
I'd recommend a system with the T6600 CPU - the M600 will be noticably slower, even at the same clock speeds.
@ JamesUCF - You don't NEED hardware virtualization support in order to run Windows 7 XP Mode (which is basically Windows Virtal PC 2007) - it just helps. You can still run XP Mode on a computer that lacks a processor with virtualization hardware assist.
Hardware virtualization support is nice - but it's generally an enhancement, not a requirement. Unless you're using Xen or something. You DON'T need it for XP Mode or Virtual PC 2007 or 2005.
How often are you going to need to run XP Mode under Windows 7? Even without the hardware-assisted virtualization support, that T6600 will run faster than that M600.
In general, the Turion II mobile processors are similar to the Athlon II desktop processors, but a little slower at the same clock speed. The M600 is roughly the equivalent of a Core 2 Duo T4300, so if you can get a laptop with the T6600 at the same price, I'd go for the system with the T6600.
Once again, go with the T6600.