Milest, the + stats compare the player to that season's average, so they are very relevant to a single season league such as certain theme leagues that use players from one season only or progressive leagues that use single season stats.
The # stats compare the player to the overall baseball history average 1885-2015, so these are more relevant to Open Leagues. The + ratings won't help you that much in those leagues, because your pitcher from 1975 or 1966 is not facing batters from that same era but rather from every era. So you need to know how they rate compared to overall history
An example: since 1930 was a mega-hitters' season like 1999 or 1894, the best pitchers from that season will be very good compared to other pitchers from the same season, but the batters from 1930 will also hit well against them and so perhaps their OAV is a good one all things considered, and if they were really good, might have a an OAV +140 or WHIP+ 140. But when up against batters from other seasons, if their best is still only average compared with the baseball history average, then they will not do well. On the other hand, if they bucked a trend, such as 1999 Pedro in a mega hitters' year, they may be world-beating compared with the overall baseball average, since their official stats understate how good they were compared with the time in which they played.
Grossly oversimplifying, the deadball era pitchers that d_rock97 refers to have numbers that when compared with the overall history of baseball (the # stats) are still very good, hardly any home runs for example, so they are very tough in OLs or leagues with players from all eras.
So, in general - single season league, go with +, multi-season and Open leagues, give priority to # stats.