Interesting, I am just dealing with this topic because my father asked me for a replacement for his old SHARP.
My findings (but I am not 100% sure):
Casio: FX-115ES and FX-911ES is the same calculator with different labels for different markets.
There is also updated "plus" model. The "plus" model has faster MCU, few more functions and slightly different look. There is also a special "DE" variant for Germany with more functions.
I expect that other, country-specific versions, exist which may or may be not identical.
Version FX-570ES is again identical but without the solar cell.
The most recent model of 115/911/570 series is the "X" version. It has again modern look, some extra functions, new menu. Also exists in "DE" version which is the most advanced, but however only with German manual and menu.
Sources:
https://de.wikipedia.org/wiki/Casio_fx-991EShttp://www.numericana.com/answer/casio.htmSharp:The answer on that from Sharp is "EL-W506" and the updated model "EL-W506X". The X version comes in different colors. I got it from amazon and I am going to return it immediately. First, I got the "violet" version. Second, the mechanical construction and overal look and feel is terrible. Over and above the front face, including buttons, is "piano black".... (do I need to comment on this ?)
.
In the US, this calculator is sold as W516XBSL.
Sources:
http://www.sharp-world.com/products/calculators/sc_calculator/index.htmlhttp://sharpcalculators.com/index.php/products#!/EL-W516XBSL-Advanced-Scientific-Calculator-with-WriteView™-4-Line-Display-&-Solar-Power/p/44939237/category=11711077
Isn't that madness ?
I would go with Casio "X" version (non-de). However I can not find this on Amazon.de so I may buy him EL-W506 (the old model) or Casio ES+ variant (that one is also available here in English).
I hope it helps.