Typically the Crit chance is of more benefit, but I would suggest you check out the Optimal Mana boost thread and determine if this extra 2% will help boost you to charging with less tiles: Optimal Mana Bonus Numbers for Every Mana Speed (Now including Ninja Heroes)
Sometimes cutting off that extra tile is great (ex. Moving an average hero to charging in 9 tiles) but with a slow guy moving from 12 to 11 tiles still means you are usually charging by matching sets of three so it is not as huge a difference.