Not that this decides whether you should buy it, but the Masterworks will look different from the omnibus version - those versions were recolored in a slightly more modern way, while the Masterworks will look more like the other Masterworks color reconstructions.
Compare the version of issue #158 in the last Masterworks volume to the way it looks in the Omnibus, and then decide if you think the different coloring approach is worth investing in the new volumes (I'm not sure, myself).