You'll be fine on an 18, especially for longer rides.
I'm a bit shorter than you and ride a medium Spesh...my SX, Big Hit and Demo 9 were all this size and were perfect.
I used to like riding dinky bikes, but got hacked off with banging my knees on the bars when sprinting and then discovered that a bigger bike was easier to pilot and much more stable. The large SX / Enduro feels like a barge though.
It the Patriot is a bit porky for longer rides I'd steer clear of the SX as they're heavier, especially the later ones with more travel. Mine comes in about 32lbs but the 36's are light, as are the Atlas cranks and Superstar pedals. With Z150's, Diabolus cranks and Azonic A frames it was the best part of 37lbs!
2015 Stanton Ti Switchback
2006 Specialized Demo 8
2013 Specialized Allez Sport
1999 Santa Cruz Chameleon
1994 Cannnondale Beast of the East (rebuild still in progress)