It’s really up to you which version you want, but here are the version differences to help you decide.
Brilliant Diamond exclusives
Caterpie
Metapod
Butterfree
Ekans
Arbok
Growlithe
Arcanine
Seel
Dewgong
Scyther
Scizor
Elekid
Electabuzz
Electivire
Murkrow
Honchkrow
Gligar
Gliscor
Entei
Raikou
Suicune
Larvitar
Pupitar
Tyranitar
Ho-oh
Seedot
Nuzleaf
Shiftry
Mawile
Zangoose
Solrock
Kecleon
Cranidos
Rampardos
Stunky
Skuntank
Dialga
Shining Pearl exclusives
Weedle
Kakuna
Beedrill
Sandshrew
Sandslash
Vulpix
Ninetales
Slowpoke
Slowbro
Slowking
Magby
Magmar
Magmortar
Pinsir
Articuno
Zapdos
Moltres
Misdreavus
Mismagius
Teddiursa
Ursaring
Stantler
Lugia
Lotad
Lombre
Ludicolo
Sableye
Seviper
Lunatone
Bagon
Shelgon
Salamence
Shieldon
Bastiodon
Glameow
Purugly
Palkia
source
It doesn’t particularly matter which starter you pick, and it’s almost purely opinionated. However, we do have IGT threads, which you can take a look at for motivation for your team.
I recommend you pick a starter that provides type coverage for the rest of your team if you don’t have a favourite. You can get the other two in the post-game.
In conclusion
I recommend you pick whichever game has the Pokémon you like the most, and the starter which you like most, or the starter that provides the best coverage for your team.
For your Mew question, please refer to here or repost as a separate question :)
I hope this helps!
