A wattmeter to measure static performance is the minimum you should have when dealing with big power electrics. Every change should be tested unless you have previous knowledge of the change. So if you change to a prop you have never ran before as a bare minimum it should be statically tested to make sure you are not going to overload something.

Generally speaking the static load will be higher than any in-flight load, so with that in mind a static test should ensure you do not exceed the power limitations of your setup.

Data logging is the Cadillac and shows you so much more information about what is really going on. What the minimum pack voltage is, where it happens in the flight, what temperatures are during the flight. Temperatures can peak dramatically on landing approach, I have seen battery temps rise 20+ deg C from the last maneuver to touchdown, with true in flight temps being barely above ambient (5 deg C). This is info you cannot get from a watt meter alone or post flight temperature checks, and can really help in optimizing your cooling if you have problems.

I personally like the standalone loggers because of their portability between models and also to the computer, I can bring the logger to the computer, and don't need to bring the computer to the logger. Also if you are going to be using uncommon motors/controllers/batteries I think a logger is almost a must have.

On a 2 meter pattern plane the weight of the eagletree micrologger is pretty insignificant 60g-70g or so.