Which guide did you use? I did hear about the sea sickness issue...but when I did the Captain Andy's on Kauai I was fine but maybe this is different...
Would you recommend a guided tour during sunset?
Yes, it was a great experience and would recommend it. I think sunset is better since you get both the daytime and nightime experience.
As for the guide... I hesitated before paying for one. I think the going rate is roughly $100pp to join a group of strangers for a walk. That seemed exorbitant to me. They claim they have easements, etc and it's not legal to go without a guide. I doubt it matters much - our guide did have us park on private property with whom they had an arrangement, but from there on it was over the lava. And I somehow doubt they got permission from each of the private landowners whose land sits under all that lava.
Despite all that, it was our first time on the island and we didn't have time to waste, so I went for it just to avoid any surprises. I found someone that did it for $60pp or so (will try to dig up the number if you want it). And I don't regret it. But if I were doing it a second time, I would look again into the possibility of doing it on my own.