Hello~ in short, sideA should be played first. sideA and sideZ are two parts of a single game meant to be played in a certain order.
sideZ assumes you've already played sideA and it just won't be the same experience without it + you'll miss major key parts lore-wise. honeybee black purposefully released sideA first before sideZ.
sideA has the longer common route and is the introduction to the game, its characters, and world building. Missing out on it also means losing out on a common route with one of the best found family and character development you can see in otoge.
It's not a matter of either or, there's a reason the last routes are locked in the two games respectively. If you played Virche for example, it's pretty much the equivalent of skipping everyone(and Yves) and heading straight into Le Salut(yes it affects the experience that much). I don't want to bring up game breaking spoilers to explain the need for the play order, but sideA ends on a cliffhanger, normally you're not supposed to have interest in Haruma except when you finish the epilogue (and the common route ending) in sideA.
Iirc, the switch versions are pretty much the same content as the first press release but with a QoL improvement like skip to next choice.
