Personally I would try the squat PR first, mainly because it would take more out of me. Plus its a good way to get the body fired up.
If I had the choice though, I wouldn't do them on the same day. I guess in this case that isn't an option.
Mobility work is good. Make sure you've got a good warmup under your belt with some good acclimation and you should be set to crank out a max attempt. Maybe do some fast triples (fast on the concentric, regular speed on the eccentric, dont bounce) during your warmup rather than just doing them at normal tempo.
The other thing I would stress is to do the PR safely. It sounds funny and sort of intuitive, but grinding out a PR can put you in a very vulnerable spot. Don't let a competition that ends tomorrow put you out for 6 months from tomorrow.
I'm not saying you shouldn't grind it out, but if you're not feeling safe or your body is saying no...rack the bar or dump it and live to fight again.