Personally I don't find any major differences between the storage capacity of sellier/rigide and retourne kellies. Also, 28cm is the max size that I would ever consider for a kelly.
So for me, it boils down to how the kelly looks. My first kelly was a togo sellier...so I thought I belonged to the camp who preferred external stitching. I liked how the softness of togo was strengthened by the external stitching...whereas the togo retourne kelly was too slouchy/casual for my liking. When I had a chance to compare/contrast 2 kellies, one sellier, one retourne, both in vache liagee (a stiff leather), I found myself prefering the retourne kelly to the sellier one...as the external stitching coupled with the stiff leather gave the bag too "hard" a look (for want of a better phrase)
So in my case, my preference for sellier or retourne is guided by the type of leather (and of coz color).
This is really a matter of personal taste. For me and my lifestyle, I tend to lean toward retourne.
I feel it depends on the size of the bag and the leathers/colors as well.
I like retourne because it can be used during the day...on some rigide bags I feel they look too formal for day.
If I did buy a rigide bag I would most likely buy a 28. That would be plenty large enough for me and yet small enough to work as an evening bag.
It really depends on what you have going on in that brain of yours.
TammyD, I am a sellier kind of girl ... I am often casually dressed but I still carry my sellier Kellys. There are of course some days when I have loads of errands to run, if I want to carry a Kelly, I would pick the retourne. (but most times, I would pick up my 30cm Gold Birkin on such days)
I prefer the sellier because I like a Kelly to maintain its form over time.
I would think about it in terms of both look and feel. I like the way sellier looks but prefer the retourne feels. I also find the retourne easier to get in and out of.
It also depends on the leather. My Kelly retourne in Box feels a lot more stiff than in Togo.
I am small and a Kelly 28 retourne is perfect for my daily use.