You should cancel its evolution until it's learned Solar Beam at level 46 -- Gloom won't learn it until level 52. Once it's learned Solar Beam, you might as well use a Leaf Stone right away to directly evolve it into a Vileplume.

Is Vileplume or Bellossom better?

Although Vileplume has better stats, higher Max CP, access to better moves for PvE, the decision is not as simple as one would think. ... On the other hand, Bellossom is a better choice than Vileplume for Trainer Battles.

Should I evolve to Vileplume or Bellossom?

If you evolve to Bellossom, you'll get more points added to your Gloom's defense than if you'd have otherwise evolved to Vileplume. If you evolve to Vileplume, you'll get a greater number of points to everything other than defense.