The i5-2400 will perform better. If you are selecting between 2 PC with those components, get the faster CPU (i.e. i5-2400). It is much easier to add RAM later on rather than having to upgrade to a new processor if you find the i5-2400S lacking.
I would go with the i5-2400 and upgrade the RAM later. Win 7 really needs 4GB of RAM, but 2GB will suffice. The i5-2400 is clocked a little slower to save energy so it will have lower performance by default and it cannot be improved upon as mentioned w/o having to upgrade the CPU.