Roserade is in between Venusaur and Breloom. If you want stellar damage, go Breloom, and bulk and good damage, go Venusaur. On top of that, Roserade has a one bar charge move, which doesnt support it's slightly above Breloom bulk.
Plus, Electivire can get two moves for cheap (baby) and get Thunder Punch which can get a hit in on Kyogre before it can get off the killing Waterfall.
Based on this, I'd go for Electivire all the way, especially if you don't have a Raikou or two. (Damage bonus)