The smart guys will chime in shortly so take this for what your paying for it.  Newer tractors tend to have the most power in the 1800 rpm range where the older machines tend to be in the 2000-2100 rpm range.  On CaseIH tractors they have the APM mode where it selects the appropriate gear and engine RPM for the desired load and speed.  It ALWAYS chooses a higher gear and lower RPM than I think would be appropriate.  Based on that I'm going to say it is better to gear up and throttle back as long as the engine doesn't overheat.