Although many Youtubers say that the 25 looks best in a Bandouliere version, and 30 in a classic version, I think it mostly depends on how much you would like / need to carry with you. If you carry basically your essentials and maybe a bit more, the 25 would be good. If you need an extra space, than 30. I personally prefer the 25, as I like to have my essentials with me and an extra space for a shawl or cardigan on top.
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In my Speedy 25b (and still enough space left):
- Samorga organizer
- Toiletry 15 with cosmetics
- Mini pochette with quick-access items such as pocket mirror, lippies, etc.
- Clemence wallet
- Tiffany jewelry pouch
- Hand disinfectant wipes (in an extra plastic bag), tissues, and tic tac
- Key pouch with keys on the Dragonne key holder