what is the different between D650 and Hornady LnL in regards of ability to do a powder check? Is there one is better or more convenient to do the powder check?
They are both 5 station presses and both are equally capable of using a powder check station.
The Dillon 650 has a slight edge, imo, in that it was originally designed for powder through expander. Hornady LnL originally was released with separate expander and powder drop. Hornady now has a powder through expander that comes on the LnL Ammo Plant (I don't think it comes with the stock LnL Auto Progressive), which can free up a station for powder check.
Also performing the seat and crimp on the same die would open up a station for the powder check.