Id like to thank people who have provided advice here. After a slow start, some opinions have been formed.
Research - If Im honest, unless you break the MX Duo or someone decides to wipe out all USB ports in PC's, then you will probably be using this MX Duo for a number of years. It may be that your cheaper choices are acceptable and will last that long but the Cordless MX Duo is a great purchase and as you see on this thread, its probably the number 1 cordless keyboard/mouse combo.
Perhaps if you enlightened us on what you intend to use it for, we could tailor our advice better to suit you.
I dont know whether the extra $50 is worth it to you. The extra buttons, the recharging base and the responsiveness are all strong points about the Duo. Maybe you dont need split second movement as you dont play FPSes or you dont particularly need the extra buttons because you've grown used to 2-3 buttons and a wheel.
One thing I just thought of, are you left or right handed? The MX700 is tailored to suit a right handed person and I personally dont find it too comfortable in my left hand. I tested it out for 5 minutes just now.
Another good suggestion made would be to go to a local store where you can perhaps see the mice/keyboards in action and wrap your hands around it.
Keep us informed and keep asking questions if you feel the need.