As someone who currently has some blue hair, I would go for the purple. Blue fades like crazy (I have to re-dye mine every 2 weeks) and it doesn't fade pretty either. It fades to a grey-ish beige. I have about a week til all of the blue is gone from mine and I just have a strip of platinum blonde.
Purple, on the other hand, will last about a month (on me) and fades to a nice lilac-y shade. I would go with purple just because it's lower maintenance....and Punky Colors and Manic Panic last the longest, just so you know.