Hi tammy
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).