I cannot speak for diorama since I do not own one. Only seen and fondled it in stores. For the Felicie, I got it since it felt like a great value including two additional inserts. I was able to sell it with 100% return of the purchase value plus slight profit since I bought it in Paris. Here to why I sold it other than WOCs do not fit much into my lifestyle...
The button closure is a bit fussy to close and flimsy. With overtime usuage, I'm skeptical about the wear and tear. Based on what I seen on the Diorama woc, this should not be an issue at all.
I had the empreinte so the leather is softer than canvas. You still might not want to stuff near or over over capacity to lose the shape of the WOC so I do not think it necessarily holds more than with the softer leather.
The chain on felicie is thinner but looks like low end chains imo. However, it is light and does not dig into shoulders assuming items are not heavy which is unlikely given the small size.
Hope this helps. Go with the head for long term feasibility. The heart desires are going to hard to get pass but they come and go.