Hmm, Mega Rayquaza is probably a bit stronger and can further amp its attack and speed with dragon dance, extremespeed and nomad. He's also more experienced, though Vegeta is more skilled. Mega Ray can also travel freely into space, which is significant if it ever needs to heal up via rest or snipe Vegeta from above. Overall I think that Mega Ray edges out due to versatility and power, and being able to use the terrain to its advantage.
My main issue with the "Rayquaza flies into space and snipes" bit is that Vegeta can still A) sense Rayquaza and his movements/attacks and B) attack Rayquaza via ranged spam and wide-angle attacks, both of which are favorite Vegeta tactics.
...Beyond that, though, I overall agree with GyroNutz. Vegeta ain't gettin' that immortality from Shenron today.
While I do think that the Galick Gun, Final Flash, and Big Bang Attack could probably kill Rayquaza (provided they work exactly like the Kamehameha and Special Beam Cannon/Makkankosappo, concentrating one's power level to increase it), Big Bang Attack will miss when Rayquaza goes up into space (which is one of his go-to moves), and Galick Gun would need to be a wide-angle blast (think the FighterZ destructive finishes where a massive blast careens off-planet), or Rayquaza could snake right around it.
Vegeta's best hope of victory is to spam range and hope that Rayquaza doesn't Dragon Dance his way to a one-shot/blitz worthy AP/speed advantage.
Ray having a spammable option to boost AP and Speed means Vegeta needs to end this quickly or Ray's going to be basically untouchable and steamrolling. Even with impressive attacks on Vegeta's side, it seems more likely and even easier for Ray to get his speed advantage back despite the equalization.