are they equal or is one worse then the other in evaluating defense.

For all practical purposes they are equal. Some Errors allow the runner to take an extra base, which is not as costly as Errors that turn an Out into a Hit. I don't have any stats on how often this occurs for Errors and Minus Plays. If there's a difference then one stat would be slightly less significant.

I looked into this a little bit a number of years ago. My recollection is that on average, for a given position (IF vs OF) errors cost just slightly more outs (obviously), but - plays cost slightly more in terms of bases. The differences in bases may not have been statistically significant. Given that outs matter more than bases, I think it's fair to say that errors are more costly than - plays, but I think the gap is pretty insignificant. I generally just add them together if I'm looking at a performance history, i.e. + plays - (- plays + errors). I do suspect that one could gain a small edge on the field by properly weighting all of those groups of plays, but I'm not sure it would be a big enough edge to be worth the effort.

Aren't minus plays batted balls that are not turned into outs because of lack of range? In other words, batted balls that should be turned into outs but get through (IF) or fall in (OF) for hits? And plus plays the opposite? So a team with poor fielding range will cost their pitchers hits & runs, and cause them to throw more pitches?

thank you guys.....ive always agreed with all 3 points made.
i subtract good plays from errors where errors dominate for a net number.