Arceus was more self contained maps while S/V was one giant one. So Arceus was able to build its maps more like video game levels, with a lot more clear differentiation and individuality to them. S/V was more generic open world (with many regions straight called North 1/2/3), but way more packed with Pokemon in each space (when the popin wasn't ruining it).

So in theory it would sound like Arceus was easily superior, but I found that S/V was a lot more worthwhile to explore. Once you've stopped trying to catch X and Y species, you'd often end up just running by entire areas. Wherein S/V I was constantly finding hidden things, and often times incredible Pokemon tucked away in them.

Which was kinda the point I was getting at up there. The environments themselves are clearly a step down (and almost certainly that bland because of their terrible laggy coding necessitating blandness to run), but there is a lot more to actually find in most locations.