On short haul market, I'd suggest Airbus A320. They are:
-faster than 737
-(with certain engine) having a longer range than 737
-more flexible on the configs than 737 (34 combinations VS 8 combinations for the year 2005, perhaps more?)
-generally more fuel-saving than 737
-more competitive on the most important thing: money issue, they are cheaper than 737
The one and only one point that makes me disappointed is: carrying less paxs than 737 (for each competing model pairs).
Why choosing 737 then?

But on the long haul market, I love Boeing, especially 777. Boeing does do a decent work for the long haul market.