I like the looks of the 300--but if I were renting, I'd be happy with either, and wouldn't base my decision on it.
Therefore, if it were my business, if the cost of the two vehicles is the same, I'd make the decision based on service/upkeep cost, vehicle life expectancy, and expected residual value when you're done with it.
Any chance the Lincoln is available without the black stripe all the way down the side? I think one reason the 300 looks better to me is the clean, all-white look...