Is there a difference between "no clean" and "no clean, water soluble" solder? I am being told there is no difference but I find the "no clean, water soluble" harder to use.
AFAIK, "no-clean" refers to the flux in the solder not needing to be cleaned off a board, whereas "no-clean, water-soluble" means it can be cleaned with just water rather than requiring harsh solvents. The only difference is the composition of the flux, so if you don't particularly care about cleaning, use whichever you find easier to use.
No-clean flux is very difficult to get off of boards, and is not recommended if you need the boards to be 100% clean (ie. if you need to apply a conformal coating to the board). No-clean water soluble means that you can clean the flux off of the board with a water wash process if needed.
If your boards are for hobby or light industrial use where no-clean flux residue is OK, I recommend just using no-clean flux. If you plan to apply a conformal coating, I would recommend using no-clean water soluble flux and cleaning the board with a wash process before applying the coating.