Giga Drain isn't great in Gen III, with 5 PP and 60 BP. SolarBeam's 10 PP and 120 BP make it better suited to longer battles, such as against Juan, where Sunny Day + SolarBeam will take less time than clicking Giga Drain over and over. You can still use Giga Drain for HP recovery if you're playing without the use of items in battle, but SolarBeam will often be stronger for longer battles.
Vileplume doesn't get much in the way of coverage, so you would generally be perfectly fine using both moves. Giga Drain is better suited to random Trainers that you don't have to waste time on with Sunny Day or charging turns.
