I agree that diamonds pointed up are easier to execute than flats, so that's where I go.

I am predominantly a 20 lpi guy; 20 looks good on most anything. I will back down to 18 or 15 if trying to rehab or replicate an inexpensive gun.

For a competition gun, however, I prefer 24 lpi. The grip area on a gun used for targets gets a lot of skin/glove contact in a short span of time, and the finer checkering feels considerably softer in the hand. Looks great, too.