I've organized events at the Sheraton Hotel & Towers, Renaissance, Four Seasons, Drake, Fairmont and Hyatt.

If your meeting is ~150 people, you'd be the main event at the Renaissance and Four Seasons. There would probably be other, larger events being held simultaneously at the Sheraton, Drake, Fairmont and Hyatt. (Some events would prefer to be the big fish in a small pond, so I mention it in case that's an issue.)

I've had excellent experiences at the Four Seasons, Renaissance and Sheraton. I had minor problems at the Hyatt and Fairmont, but nothing that would keep me from returning to the venue. I had a lousy experience at the Drake, and wouldn't return.

Most of these hotels will discount the cost of your meeting room if you spend enough on food and/or sleeping rooms. Since at least half of your guests are coming from out of town, this is an important factor to consider (assuming they can be relied on to stay at the hotel of your choosing).

I've also been involved in events at Navy Pier (no sleeping rooms, but lots of meeting space) and Germania Place (again, no sleeping room, but flexible meeting space). Particularly if you're looking for flexible raw space, Navy Pier is a good option. In both instances, there are decent hotels close to these locations, but you may need a shuttle to move people between the event and hotel.

I'd recommend looking at the Chicago Convention Bureau's calendar of events (http://www.choosechicago.com/Pages/default.aspx) so you can avoid planning your meeting on dates when major trade shows are in town. (These are almost certain to drive up the cost of your sleeping rooms, though you may be able to get a deal on meeting rooms if the hotel feels as if they're going to have a tough time otherwise renting the space.)