MPPT buck converter vs Ordinary buck converter?
The MPPT converter will probably have a significantly higher power draw for its own circuitry that can waste more than any gains from power tracking - even the 5-10mA quiescent current of the LM2596 might be worse than the gains over a simple low quiescent LDO.

Since in this case there is a significant voltage difference between your panel and the load (3.2V?) a switcher is almost certainly better but that might not be the case if the panel voltage matched the battery voltage better.