We have been to both and enjoyed both. Pushcart is a little more upscale, being located at RockHouse Resort on the cliff right above the caves that the CN and CSA Cat Cruises usually go to. 3 Dives has great jerk and live music quite often. I would recommend either/both. If I were trying just one... If you want a nicer place go to Pushcart, if you want a more local Jamaican feel then go to 3 Dives.
All the best!
We have tried 3 Dives and we loved it. It's making my mouth water just thinking about their jerk chicken. If you like spice ask for their spicy hot jerk sauce, but be careful, lol. It's really worth trying and you will probably want to go back a second time.