In the long term, more ram is always better than faster ram... unless you have more ram then you are going to need. But eventually the ram fills up, especially as time progresses programs are made to take up more and more ram, so the more ram will be more useful and valuable.
However if you have plenty of money, don't care about resale value, and switch hardware very often (every few months) as new hardware becomes available, AND don't do anything that fills up 4GB at the moment... than you can get greater speed with it.