The 35 kelly and 35 birkin are very different IMO -- birkin holds way more. So it really depends on what you will use it for. Both would be beautiful. I have rouge h 35 birkin in clemence (my avatar) and find it very useful and easy to wear.
A 35cm kelly is very different than a 35cm birkin, esp comparing red box. I like box leather in a 35cm birkin but not in a 35cm kelly. At that size kelly, box leather can make the bag look too briefcasey. A softer and more supple leather will lend a softer eye to the kelly. GH is perfect with box leather in either bag though.
the two bags are completely different. IMO, the kelly is a handbag & holds as much as my 30cm birkin. However, it's such a beautiful beautiful bag- so classy & so delicate. Makes me feel all dressed up even when I'm wearing my jeans & tees. My 35cm birkins are definitely more of a tote bag. It holds 2x as much than the 35cm kelly. I absolutely love the 35 for traveling or running around town with my toddler b/c I can stuff all of her stuff in there along with my purse organizer full of things.
While I don't own a bag entirely in box leather, I think either would be fabulous. I think it really depends on how you are going to use the bag. Good luck!
I think a birkin is much easier to get in and out of, and it holds more. A kelly has the great shoulder strap option. If I had the choice, I would get a 35cm birkin, and a smaller (32 or 28cm) kelly for lighter, casual days. Or a box kelly for special occasions.
While I would love either, I think in box leather I'd go for a Kelly. They are just so beautiful! In a Birkin I would want a more casual leather like Togo or Epsom. Not to say that a box Birkin wouldn't be fabulous too! But I have a weakness for Kellys and given the choice I'd vote Kelly!
Having a bag that holds more isn't a selling point to me now. I can load my 35cm Birkins until my arm feels like it's going to break! I'm into Kellies more now for the different look, lighter weight and the shoulder strap.
But choosing a bag for you to buy is a tough one. It's very personal! Maybe if we knew what other bags you have currently, we could help you round out your collection? When do you feel you would carry a red bag - formal, career or casual?