The 737-800, like all 737s, is a narrow body aircraft. Generally, narrow body = one aisle, and wide body = two aisles. I haven't flown a 737-800, but I have flown LAX-MCO and LAX-ATL on a Delta 757 (same cabin width, but longer fuselage) and I've found the seats to be a bit narrow for long flights, a common complaint with the Boeing narrowbodies (the 707, 727, 737, and 757 all have the same fuselage width). It also depends on the individual airline, but mostly on seat pitch (distance from the back of the seat in front of you to the back of your seat).
I would fly a 737-800 ORD-LAX if it worked out best for me, but if it came down to a 737 vs a Boeing widebody or any Douglas or Airbus aircraft, all else being equal, I'd probably choose against the 737.