Naturally, 70 def triumphs over 40 defence, however the nature of the account in terms of performance mainly depends on where you'll be pking, whether or not you're in a clan, and what style of fighting you chose to endeavor in, among other things.
At 40 defence, it wouldn't be the wisest of decisions to journey into deeper levels of the wilderness alone, as you may encounter more capable PKer's in terms of stats.
That being said, 70 defence gives a lot of PKers a disability in terms of having so many options. An example would be a 40 defence PKer fighting a Dharrock's PKer. My personal opinion, only supported by playing the game since 2002 and having seen and done a good bit of PKing, would give money to the rune pure based on versatility of their styles, mainly their advantage in being able to utilize a number of attacks, whereas a Dharrocker has to take chances with a lower number of HP in order to hit higher with the Dharrock set special.
In all, I'd go from 40 defence pking for a while, to 45 (Fremmy Helm) to 70, gradually in order for you to get a first hand experience on the type of PKing you enjoy, and the pro's and con's.
