It depends on what you are looking for and how much time you are willing to spend farming for the vanguard/crucible marks that you need to buy them, because weapons are usually more expensive than armor, unless you are buying a helmet which is quite close in price. If you want to level up past 25 i would recommend saving up for armor but if you just want better quality weapons that is the choice for you. My perspective is that you should buy the armor first to level up enough, then save up for the weapons.