All other things equal, connecting at DEN is much better than connecting at ORD.

There's much less chance of a weather delay at DEN. DEN is much less crowded. You have a much smaller chance of having to change concourses in DEN.

I'd avoid LGA-IAD like the plague - IIRC that's all RJs, and on that route the RJs get delayed and cancelled a lot.

If you've got the spare time, look into JFK-LAX-SEA also.