i say quest 20 defence [that includes doing what lies below]
initiate pures unlock an amazing amount of new items/shortcuts etc. that make being a pure that much easier. i say go for the rune gloves, the dagon'hai robes and the sick as hell magic secateurs that will make your life so much more enjoyable haha.
I'm pretty sure it doesn't make a significant difference (You're level will be the same at the end of it all I believe.) So you may as well stay @ that. Go quest the def if you feel it's needed, but don't forgot the prayer and the rune(Or higher?) gloves.