the flame is adjustable for the amount of water pressure, you can tweak it a little to increase or decrease flow rate and btu to get the perfect combination of temp and pressure at the faucet….
 
personally i would go with an 8L, that way you can absolutely gain maximum pressure and temp easily….. only a few more bucks, and operating cost is negligible due to the fact it only runs when you need it…..
 
if you go on ebay, you can find stainless units that will have the HD grinning, and they are “plumbed” for external venting to begin with