I see by your OP inquiry date & others posting entry dates, that I am late to the party on this one...
If I were to purchase only one Flashbender I would chose the L(Large), rather than the XL (extra-Large); a slight tradeoff for the L’s more compactness, without giving up too much of the added benefit of the XL’s more wrap around light effect (e.g. what I would call an “octabox” light distribution effect) & slightly softer light quality in the XL than the L .
If I had a 430EX, I would get the L.
L- if applications & or subjects are dynamic with changing areas within a location.;such as events or weddings etc., or want a quick & easy yet quality portrait - easy to stuff its 11" fabric length in a camera bag or waist-belt system.
XL- if applications & subjects are static AND you don't mind "hauling" the 16" long fabric.
Apply the 80/20 rule - to what you will be using it for 80% of the time.
As I have found with any photography gear: if it’s compact & easy – it gets utilized!!!
[ask any tripod owner who likely bought based on specs rather than practical use. Yes I admit going down that iterative process too, Manfrotto, then Gitzo (thinking it was the panacea), only to discover that I like Sirui the best for performance/weight tradeoff for my applications].
When the Rogue Flashbender was introduced to the market a few years ago, I initially before using/testing prejudged (unfairly), and initially categorized it as just another gimmicky product idea that many that have littered the photography market over the decades. Incorrectly thinking small product = small harsh light right? However, once I tried/purchased one shortly after around the time the product was introduced (hoping to solve my need for an effective & soft yet compact speedlight modifiers), I quickly changed my mind – yes, a paradigm shift!
I happily find the Rogue FlashBender’s (& also Rogue Grids) hallmark features of compact & light-weight effectively convenient; especially for location applications (& for occasional location applications via air-travel). The Rogue FlashBender’s satisfy 2 (QUALITY & DIRECTION) of the 3 elements to photography lighting: QUALITY, DIRECTION & color.
Properly & creatively applied, I find it enables the art AND the science of photography/lighting.
I LOVE the FlashBenders!!!
(I extensively use the Large; in contrast to the XL which only use selectively, due to its size relative to my 6-600EX-RT's size, only have two XL’s folded narrowly for out-of-studio quick strip boxes, to create on occasion, selected drama in a desired image. As KOZ21 kindly pointed out with the XL, I too reverse the speedlight).
Sometimes less is more… (the world isn’t flat – only the Rogue FlashBender is). Just saying… there is sometimes a great view once one dares to look over the edge.
No right or wrong, everyone has to decide what tools work best for one’s own INDIVIDUAL style and INDIVIDUAL photographer market differentiator.
Just one view…
Good luck in whatever light modifier you use.
(P.S. No I didn’t drink the Rogue kool-aid, I prefer water).