If you're leaning towards blue/turquoise, then I'd go the blue, because if it's anything like the blue I put in my hair, it generally fades to a nice turquoise anyway, so you get the best of both worlds.

If you're leaning towards the blue/purple, I'd go purple, because it generally fades to mostly blue with a little pink/purple still, which looks really awesome.

My tips are blue at the moment - I redyed them last night.