If you’re posting this, you likely already have an idea of which one you’d like to go for next but you’re wondering how to validate that decision.
I agree with the other posters that you have to ask yourself some questions:
1.For what occasions will I use the bag? Casual day time events? Casual evening events? Work? Errands? A Birkin 35 and Kelly 28 are larger sizes so I lean toward reserving these for informal use. As a very general rule, the Birkin is more of a tote and works well when you need to carry larger (but not necessarily heavier) things. I’d use it for occassions where you’d typically carry a tote. You could also carry a Kelly retourne for these more casual purposes.
A Sellier Kelly, because of its top handle and trapezoidal silhouette, has a classic connotation and is better suited for scheduled events / outings. I’d carry a Birkin when shopping, running errands, to work, play date, meeting a friend for brunch, etc. I’d carry a Sellier Kelly when meeting a client for lunch, shopping, meeting friends for happy hour drinks, group brunch, and daytime events where I received an invite. This is all completely my opinion, YMMV.
2. What’s my personal style and what message am I trying to send when wearing this bag? This question is arguably more crucial than the first. For classic vintage silhouettes, structured outfits made from woven fabric and understated looks, I favor the Sellier. If you want to flex, go for another Birkin! Birkins and Retournes also go really well with more casual outfits made from knit-based fabric (most contemporary clothing).
3. What are my proportions? People talk a lot about height here but width also matters. Would a Birkin 35 / Kelly 28 sellier / Kelly 28 retourne make you look bulky or streamlined? Do you want it to blend in with your proportions or make a statement by contrasting with your proportions?
I wrote a lot, but I hope it helps. I left out considerations on leather and hardware, but these also matter.