According to Broadcom, the main difference between the two processors is that the 4709 has a 1 GHz clock speed per core, vs. 4708's 800 MHz. The 4709 also supports faster DDR3-1600 memory and has "some other I/O" differences.
I assume you will go for DD-WRT or other third party firmware..
Therefore, if you are confident and skilled with firmware flashing, it is suggested to select Netgear..