EF L vs RF Lens for R6Mkiii

I'm looking to build up my glass to last as I just upgraded from a Rebel t5 to the R6mkiii. I have an EF adapter already for some of my old glass, but is it worth still investing in EF L glass or is a basic RF lens better for my camera? I travel a fair bit so sturdiness is a bit more of a priority.

For my main example, I found listings for a Mint-Condition EF 20-70 F2.8 L for around $800, then there's the RF 28-70 F2.8 for around $1,100 new, and the further out of budget L version for $3,300. Would I be getting "worse" image quality with the EF L because of the adapter? Or would the biggest difference be the autofocus?