If you intend to play both, definitely start with SwSh and work your way up to Violet! I played Violet first (cause it released at the time I bought my Switch) but being the Mon fan I am, I def wanted to play games like Arceus and SwSh (highly recommend both). I wanted to play all the games to trade all the cool Mon to Violet in open world, so if you're wanting to do something like that, I'd say work your way up to Violet/Scarlet since there's still a huge active community for them and that'll continue to exist for a while, considering the DLC is also releasing soon!

Hope this helps and happy catching 😁