For an in-ground pool costing $30k + then the cost of a heater is minor in relation to the overall cost of the project.
For above ground pools that cost in the $4-5k range, the cost of adding a heater is significant.
If your budget allows, a 400k btu heater will certainly heat your water quickly. The down side to the larger units are the higher up front cost and the higher install costs.
For example, a Pentair master temp is around $2300 + install.
The cost of the heater + installation is probably more than the entire cost of your above ground pool.
The heaters specifically marketed for the above ground market are lower in BTU output and will take longer to warm your pool; however, they will be more in line with the price of the total pool project. Pentair minimax 100k btu is around $900 + install.
http://www.poolcenter.com/heaters_purex ... olstor.htm
Hayward minivolt above ground units are $800 + install.
http://www.poolcenter.com/heaters-haywa ... ground.htm
The install on these smaller units will be less than the larger units as you are able to use smaller gas lines, 1 1/2" connections for water and the electrical connections are 120v plug-in.
Price it both ways and let us know what you decide.
PSG