Honestly, it depends on what I'm in the mood for.
Mains are great for bossing, questing, and doing whatever you want without the risk of experience in an unwanted stat. Jagex is starting to add quite a few high-combat things to the game, so it's good to have a maxed main handy when I feel like adventuring with the content.
Pures are great for PKing (obviously), skilling, and getting achievements (Such as Fire Cape, or a Slayer Cape). I personally prefer playing a pure, as I deem it more fun and challenging. There's a lot more things to worry about, and I like being a bit under-powered in the world. A sense of accomplishment is felt when you meet a challenging goal.
I have a 1 defence pure. It's not an amazing account, as it's still in the works. It's 68 combat, with a Fire cape, and I am in the midst of questing it so I can use the Dragon Scim, have Ancients, and god books etc. I've tried all the kinds of pures, and a 1 defence, 60 attack is definitely my favorite.