Flying versus TGV to Nice: It seems like we need to hear from the OP whether they're actually spending time in Paris. The post is titled "Advice on trip to Paris", so it's reasonable to assume they're going to Paris itself, not just transiting CDG.

If they were just transiting, not stopping in Paris, I'd probably lean towards flying. That said, I'd consider instead flying to FRA or MUC and then connecting to an LH flight to NCE. Unless there's a train from CDG itself, getting to Gare de Lyon (where I think the TGV to Nice departs from) is painful on the RER or by taxi...and would itself require a considerable buffer of time.

If staying in Paris, even for one night, then the TGV seems like the no-brainer. Way less stressful than getting back to CDG or Orly and checking in and flying.