That will depend entirely on your needs. I think the overlap is huge and, therefore, most of what you might want to do with one can also be done by the other. I could play you 100 different sounds and unless I was using a specific feature of one or the other, e.g. Obsession's step sequencer or the BP filter in Eight Voice, you wouldn't be able to tell which instrument it had come from. Such is the nature of synthesisers.
I think most people would disagree when it comes to this particular emulation. Try it for yourself - take a lush preset like "Belief" (in the Basses category) and try to change it into something more like a bassline - fast attack, shorter decay, lower cutoff, more modulation and a little more resonance. You either have to adjust each voice individually, which is a punish, or group them together and lose the fatness that's been patched into the original preset. OTOH, with Obsession you can dial that fatness in with a single knob if you don't want to go in and do it voice by voice. Same with bx_oberhausen - just switch on TMT and adjust the "Spread" in the oscillators and you've got your fatness way more easily than with Eight Voice.
I am finding it quite frustrating. It feels like you either have to use the presets as they are or spend countless hours trying to carefully edit them without losing the essential mojo that made you choose that preset in the first place. It sounds really good but trying to use it in a song is a punish so I'm afraid it is going to be the last thing I think to use most of the time. Add that one feature, though, and it will probably be the first thing I reach for.