there's people that actually like Massive more than FM8? that's news to me. i think the only one i've heard of who likes Massive over FM8 is Kill The Noise cuz he said FM8 was too difficult for him. i heard that from someone who said he said it, so don't quote me on that. i don't think there's any more work in one than the other, they're just different and the amount of time is spent in different places. personally i feel like if you put them side by side without additional plugins, etc, that FM8 has a MUCH better sound to it. yeah they sound way different, FM vs wavetable, but as far as sound quality goes i say FM8 without a doubt.
Massive's sound quality leaves alot to be desired IMO. i love it's capabilities, for example to get movement in Massive you have tons of modulation and wavetables to get movement in your sound. if you don't mind all the processing, EQ'ing, plugins, layering to make Massive sound good then it's a great tool. so I usually spent most of my time trying to get my Massive patch actually sounding good. it seems with Massive that less is more, try to keep the patches simple and not necessarily use all 3 oscillators.
FM8 doesn't require as much processing after you finish the patch you're working on. the time-consuming part of FM8 is all the experimentation you do in order to find a good sound because frequency modulation can be very touchy when you use one operator to modulate a chain of others, etc. FM8 has a different point of view when getting movement into your sound - you can automate the amount of modulation between operators and also draw out the envelope for each operator, so some operators can come in at a curve then dip in and out and release... or you can draw a formant envelope. a small change can make a big impact so coming up with the initial sounds can be tedious. but the more you use it, the better feel you have for it. so i spent most of my time in FM8 just programming the patch then it requires minimal work after that.
anyway that's my 2 cents based on my time spent with them, others might disagree if they have different experiences with them