I'll give you the basic facts and then my honest opinion.
F2P has a much larger population and the PvP is much more apparent there. However, if you haven't previously spent much money or spent a lot of time farming runes/Pat5/strongbox sales/battlemounts or whatever else F2P has to offer then be prepared to spend a pretty penny to be able to handle yourself in a PvP scenario. You'll be facing people who have spent... a "considerable" amount of money for many people, just so they can demolish you before you even realize you've been targeted.
P2P has a small and growing population. We had one PvP skirmish pop the other day; it only popped once and hasn't popped since, and that was the first time it has popped in a very long time. Most might consider it a sad achievement, but we're quite proud of the fact that we were able to get it to pop at all. At the start of the patch we had maybe 7 or 8 people on at any given time; right now it's at about 14-17 on, so we've essentially doubled our numbers.
My suggestion to you would be to take advantage of the free subscription month now and see how you like it. If it's not your cup of tea then you essentially haven't lost anything except for a month's worth of play time, which means very little on the F2P server in the grand scheme of things. On the other hand, time spent playing (versus amount of money spent) is essential for subscription server, and now is the best time to get started.
Good luck, and if you happen to roll on subscription server (league side) you're welcome to come join us in Void.