The diversifiers are hedge fund styles that will provide a greater degree of diversification because the correlation will be low and potentially negative during "bad times", those periods when equities do poorly. The diversifiers include managed futures, which have low correlation during crises, global macro which will take large beta bets from both the long and short side of the market, and equity market neutral which reduces most of the market beta.
Substitutes look to lower volatility and add alpha. Diversifiers look to dampen volatility through dynamic beta. This simple framework makes the question of choosing hedge fund styles very simple. Do you want a substitute for market exposure, or do you want diversification protection? Answer the macro question and then focus on the right managers within these two major classification.