Since you want it to last four or five years, I think you need to get a 15" MBP with HD display option and the dedicated graphics card and 512 MB VRAM.
The CPU on either of the MBPs will be fine for four or five years. The real bottleneck in computers lies with the drive and drive controller. However, you can upgrade that later when SSD prices drop. The next important feature will be the graphics. The problem with the 13" is it has an acceptable graphics card for now, but for the long haul you really need a dedicated graphics card with at least 512 MB of VRAM. The last thing you should worry about is the RAM. You will get 4 GB of RAM now which will be plenty for the next two years. In two years you will be able to upgrade it to 8 GB of RAM for probably $150. So buy the high-end 15" MBP now. In two years, upgrade the drive to an SSD and to 8 GB of RAM.
Computer CPUs today are plenty fast enough. The real limitations come with the drive and drive controller, graphics cards, RAM, and most importantly software. With the 15" MBP, it should get faster over time with apps written to take advantage of h.264 for the graphics, OpenCL will allow the GPU to be well used, and Grand Central Dispatch will take advantage of the CPU cores.
If you only needed the computer for two to three years, I would say get whichever you think you would like better. Since you're making this a long-term investment, allow yourself to spend the money now to get the best overall system you can. Then in the future, when an SSD costs $150 and 8 GB of RAM costs $150, you can upgrade them. You're probably not going to use up 4 GB of system RAM right now, but apps will require more and more resources over time. With this strategy, in two years when you're upgrading the RAM and SSD it will feel like you bought a brand new MBP.
One last time... get the best MBP you can afford. Get the HD display for $100. Ensure that you get the 15" with high-end 330 GT with 512 MB VRAM. With that computer, you will have the ultimate experience throughout the whole ownership period of the computer. Also consider buying yourself a 24" LED ACD in the future. I have bought three of them for $599 refurbished, as they turn the Mac notebooks into instant "desktops."
Good luck with it... enjoy it whichever way you go.