If you are going to get both eventually, I would start with Diamond. In my opinion that makes more sense in terms of story and gameplay transition. Plus, there will be a limited time distribution in April for Darkrai in Diamond.
Brilliant Diamond is most similar to red and blue so that is what you are probably expecting when you play.
Arceus is a lot of fun but does not feel like a typical pokemon game so if that is what you are looking for then go with brilliant diamond. However, Arceus is still a blast which is why i said both.
I enjoyed both of them. I’d go for BDSP personally. Then if you like that that, try buying Arceus down the line. The main games are closer to BDSP than Arceus.
I had the same choice and went with Arceus. So far loving it. Really didn’t care for the let’s go remakes because of the battle style they chose. With all the versatility in Arceus it has been a fun change up.
Playing Arceus and decided to try and run my whole team (under lv 25) against a wild lv 45. Of course my team is wiped out, but the great thing is… I didn’t black out. You are just stuck running around in the wild with a KO’d team. Absolutely love this change!
i understand this is super late and you’ve probably already bought one, but i’d recommend legends arceus over bdsp any day.
if you are looking for the more classic sinnoh experience, i’d say take the money you’d pay for a brand new copy of bdsp and put it towards a ds or 3ds (whichever is cheaper, both play DS games) and a cartridge only copy of pokemon platinum. to this day, platinum is still one of the best pokemon games imo and is head and shoulders above bdsp in terms of content, polish, replayability, etc. with a ds you can also grab black/white which are also phenomenal games. if you’d prefer to stick with your switch, definitely go with arceus. cheers!