from a purely practical view, it depends on three things:
1) The day and time of your arrival. If it is in rush-hour on a work day, it will take much more time by taxi. Worse in the morning (when most traffic goes into the city) but even in the evening it can be bad.
2) How much luggage do you have? The metro is not great if you have big and/or heavy bags. Especially if you need to change lines where there may be stairs without escalators. And the time when it makes the most sense to take the metro (see #1 above) is the time the metro is most packed.
3) Your comfort level with new challenges. If you cannot read any Russian and do not have much experience with different types of metro systems, you can get lost and confused easily enough.
I recommend all my visitors to Moscow to take the trains and metro, but I also ensure they are prepared...