Well before I answer anything, the main problem I have is why you are using an Eviolite on a Slowbro, which cannot evolve any further. If you were thinking that Slowbro evolves into Slowking, it doesn't - Slowpoke evolves into it.
In terms of which move, I'd say Ice Beam offers more utility overall, because it provides more coverage. However, Flamethrower is your only option of hitting Steel Pokemon. It will really depend on what the rest of the team is - if your team doesn't worry much about Steels, than Ice Beam is your choice. Otherwise go for Flamethrower.
I'd actually go for neither of them - I would run Scald over both of them. Scald is a great STAB move - it has a 30% chance to burn the opponent, which, along with Calm Mind, turns Slowbro into a bulky monster. I'd replace Psyshock with Ice Beam or Flamethrower.
