I would ask myself which one I would regret more. The one that may need time for repairs or the one that will cost me more up front.
Good point on the 350 being able to haul more and trailer more. That along with the warranty is a big plus. That can make you more money in the long run.
Of course the other side of the coin is whether or not the older one will cost you $5K in repairs, down time, and loss of opportunities due to capacity.