this is going to get confusing and difficult to compare motors from indian and chinese factories if the power ratings are made up arbitrarily, a motor will have a given continuous power rating no matter what voltage/current its tested on, calling the motors 400watt and 600watt and 1 kw but not based on a standard way of calculating continuous power rating makes it meaningless when comparing to other motors as its arbritary values ( i.e made up values).
You would have to define how you are getting these power rating values to have any meaning when comparing motors, ultimately how the motor performs is the most important factor.
What are the internal differences between the 400 and 600watt motors? are the windings different in any way?
Its almost certain the motor you are calling 400watt is the same as the chinese version which was called 350watt, but when tested its continuous power rating was in fact 250watt. So your 400watt motor would almost certainly be a 250watt rated motor if it was tested on a torque testing machine ( the graph data can be used to determine the continuous power rating), just using volts and amps isn't enough info to determine continous power rating.
Let me make a guess: your 400watt motor has two strand type windings in it, the other motor your calling 600watt should have a single strand type winding ( but almost certainly wont be 600watt rated more likely 350watt)
Personally I prefer to use max power ratings, the max the motor can handle without burning out if under load for some time, but there are conventions for continuous power ratings, which your not using, so although its nice to say 400watt and 600watt continuous power rated motors it doesn't really mean anything as the values your giving them are arbritary.
If you can give the winding resistances and number of strands in the windings then at least some comparison can be made to the chinese made motors, the point I was trying to make in an eariler post is that these motors are coming from india, based on technology/knowledge transferred from china, chinese have alot of experience with these motors over many years, india its a new thing to make these motors, so comparisons of indian and chinese motors would be quite valuable, other point I was hinting at is that there has been a long difficult process involved with the chinese bmc hub motors, likely a similar process will occur with the indian made ones. If the chinese/indian factories were in very close contact it might not be a problem, but it appears the ties are not very close.