In order to answer your question you must first identify the limiting factors in your current design. So, for example, if you need a lot of GPU memory and are running 128/128 split and with that the amout of system memory is simply not enough for all the applications then you'll get a lot of swapping. Virtual memory is much slower than real one and even more so on RPi where the SD access is not exactly lightning fast. In this case you would benefit from having 512MB model. But this is a very specific scenario.
Neither CPU nor GPU in RPi will became any faster just by having more RAM attached. There would be some difference if the RAM was not just bigger but also faster (lower latency for example). Not the case here as I understand it. In the end the important thing is how you use the hardware.
As a side note, in current firmware there's just one start file and the memory split is defined by an option in config.txt. Well, two start files, one is needed for extreme case with GPU having only 16MB assigned, but it will be selected automatically for you, no need to touch anything but the config file.