On one land, double Goblin Guide or Guide + Swiftspear is better than double Monastery Swiftspear. Double Swiftspear deals three damage by the end of turn two, compared to 6 for double Guide and 5 for Guide + Swiftspear. 17 life is a Lightning Bolt higher than 14 or 15. By turn three, opponents will have blockers or removal, so creatures rarely deal additional damage.