I would recommend both, but that is a big cash outlay - especially if you already own the hardbacks. The main difference is quantity. Sure, you already have access to a ton of monsters through the SRD, but with the Monster Manual you get all the artwork and tokens to go with them. Volo's will give you access to the stats, artwork and tokens of a bunch of new monsters and 7 new player races, but you can't beat the MM for time saving in setting up tokens for all of the more commonly used monsters.