Just do the best you can with what you have. If you want to purchase Melodyne for a few hundred bucks, i think its well worth it.
Is Melodyne's effect stronger than Autotune?
I have melodyne and i use it allot when mixing vocals. As far as being "stronger" yes and no. It depends on how you set it up.
If your chorus plugin is sounding like a flanger, then you need to set it up differently, so it sounds like a chorus. A Chorus FX can be made to sound like a flanger and a Flanger FX can be made to sound like a chorus when set up properly.