The Player's Guide and the Boxed Set are complementary but different.
The Player's Guide includes races and classes, a player's history (not the real stuff, just what it known), new skills, new feats, a geographic overview of the general info on the main cities and geographic features, the gods, new monsters, etc.
The Boxed Set is the DM (aka Judge) information. The full DM history, encounter and terrain information, weather, special substances, that sort of stuff, plus full DM info on EVERY SINGLE FREAKING village, city, town, castle, citadel, ruin, relic, island, lair and geographic feature found on the map. And there are 18 maps! The Boxed Set also includes some info on JG and the old products and how we incorporated that stuff into the boxed set so that no one has to feel that they have to go dig up 20 now out of print products to use the setting. We have done that all for you.
Basically, you could use any player's material with the Boxed Set. OA rules, Monte's variants, whatever. The Boxed Set is a setting.
Clark