Unless the bike has an extreme compact top tube, a large may well be too big for you. At nearly 6 feet with a 31 inseam I ride an 18 (see note later) and have often thought about a 17.
Note: MTB manufacturers have much more variation between virtual top tube length than road bike manufacturers do (Possibly because road bike design is more mature). Even though I ride an 18" MTB, the top tube is in excess of 23" long (virtual). Haro builds their hardtails like that with an extremely sloped top tube which provides the clearance the Stapfam points out is "comfortable". In addition to all of this is suspension sag. If the front fork sags the recomended 25% of travel when your weight is on the bike then the top tube will be at a much higher level when you climb on the bike or stop suddenly than it will be when riding.
Most recomendations are to go with the smaller size when in doubt. You will be more able to hang the but off the back of the saddle to better distribute the weight during technical descents. You will ride level ground with your arms more bent (No straight arm riding) and the bike will be more maneuverable on tight twisty trails.