The heat flux (W/m2) value is a heat transfer rate based on how much surface area you have selected for your surface parameter. The heat transfer rate (W) is definitely what you are looking for if your intention is to see how much power is moving in/out of your system. The fact that you are expecting 1274 W and are measuring 440 W is a disconcerting difference. You will have to determine whether the source you are getting 1274 W from is valid or whether the flow project itself is in some way designed to deviate from real life to give 440 W when it should give 1274 W.
use the mass flows and calculate the heat transfer with that: delta T in -detla T out * Cp for both streams and see what you get.