There is a ridiculous amount of comparison videos on Youtube for the 25 and 30, not to mention a good ol' fashioned Google search. Interior and exterior-wise they're identical - with all of the Speedys having zippered interior pockets now - and equally cavernous with the only noticeable difference being the size, although some people find getting into the 25 is a little more difficult due to the naturally smaller opening. The 30 is considered to the "Goldilocks size" since it's neither too small or too large, but just right for most body shapes and heights (my mum is 4' 11 and can wear a 30 without issue so long as it has a base shaper to prevent sagging which can make the 30 look larger and 'sloppier' than it really is).
What you choose will depend on a lot of factors such as how much you plan to carry (the 25 can hold quite a bit for its size, but that small opening can be an inconvenience, especially if you're trying to grab something in a hurry), your body type/size, personal preference, etc. I should also mention that the 25, even when full, won't sag as much as the 30 can - unless you love the sag - but a good base shaper will do wonders
Unless the savings are significant, at least to you, and the pre-loved 30 is in excellent condition then I'd personally pass and instead purchase new.
I won't lie when I say that carrying a strictly handheld design is a pure joy because it's not. There have been many times when I wish that my classic Speedy had the option of being worn hands-free, but because it isn't I feel restricted when out shopping or running similar errands and end up selecting a different bag. Spend the extra money for the bandolier version.
