Often I have had to explain to people the difference between the first and second focal plane scopes. Now Primal Rights has a nice article explaining the difference between the two. The article can be found here.
Personally I prefer first focal plane as my reticle and my target will always stay the same proportions to each other. One thing to note is that newer FFP scopes have much better designed reticles then older scopes. Many of the old FFP scopes used reticles designed for SFP scopes, so the reticle would end up being very thin or very thick on one or both extremes of magnification. Most newer FFP scopes have well thought out reticles that remain useful thoughout their entire power range.
But in the end, the important thing is to know your scope.