It's very simple. Don't buy into the most expensive upgrades with the false concept of future proofing the device. The $1800 model next year will outpace the $2200 model today. You don't really buy much in the way of longevity once you're past the largest hurdles such as discrete gpu and quad cpu. Apple does short their base 15" gpus on vram a bit. This could become an issue later under games and certain applications. Whether you upgrade two years from now though rarely has much to do with if you opted for the 2.3 or the 2.6. There's a relatively narrow range where you would choke one but not the other. At that point that it can be mostly addressed with settings adjustments, but this is a relatively rare scenario. The next hurdle beyond that will likely rule out both. Some of these options are just for maximizing power in a mobile form factor where cost is nearly a non issue. It helps comfort users who are accustomed to spending most of their time on a desktop or workstation class machine when it is unavailable.
By the way if you're in the US, check out some of the third party retailers. Amazon had a cheaper offer on the rMBP that might help you rest easier with the decision. I just don't see the slightly faster cpu as being a big motivation to go with the other design.