I live in Chicago and ride Palos often, so I have a decent idea at what trails you will be riding.
I made the mistake of getting a too-small frame that I thought I could manage through long seatposts and stems.
MISTAKE!!!!!!!!
I always regretted it until I got the right size this year. I'm 6'3", 34" inseam, and I bought a 22.5" Giant NRS. Perfect!
Advantages:
1) Not feeling cramped on the bike.
2) Not having a rediculously long seatpost and stem.
3) Front end is not obscenely light as to lift in climbs.
4) I'm much faster now with quicker steering and better balance.
Try them out and see what feels best. Have you been to Richard's? Excellent shop in Palos Heights not too far from the Palos trailhead.
www.rbikes.com Check them out. My buddy got a great deal on a Jamis Dakar there.