1) After observing closely for a few years, I feel pretty confident that (-) plays at a position are determined by comparing the player's rating to the expected ratings for that position, rather than using some absolute value of range. So, on a per-play basis, a bad defender will make a few more (-) plays in RF than in LF.
2) That said, the RF sees about 2/3 the balls of the LF.
If I need to skimp on defense I usually do so in RF-- but I think it's pretty close, and if I were facing a lineup with a lot of LHB I might change up and skimp in LF.