To get the most life out of your rMBP at the lowest price, go with the stock 2.3 GHz and max out the RAM to 16 GB. Keep the 256 GB SSD, which you will use only for your OS and applications, and buy yourself a fast external hard drive or RAID array for your work files, documents, etc.
If you only get 8 GB RAM and hit your memory ceiling, the OS will start paging out to your SSD drive. While an SSD is faster than a hard disk, it is still only a fraction of the speed of RAM. The difference between the 2.3 and 2.6 is approximately 10%, but if you run low on RAM the performance hit will be far worse than 10%. Most often the performance bottleneck in modern computers isn't the CPU - it's the disk drives and RAM.