I've tested the following:
- SCI-15C, Tom Fors (based on nonpareil (Eric Smith)
- ClassicRPN (they rolled up 11C, 12C, 15C, 16C into a single package)
- RPN-15C, same as #2, but just the 15C.
- Calc 15C (now gone from store), however Calc 16Cx and 11Cx are still there.
Of the 15Cs I've tested #1 provides the best user experience because of its simplicity, keyboard accuracy, redesigned rear view, and the big button feature. It also supports all the 15C self-tests and diags, even on-. works. It is a bit pricy, but since Eric gets some coin from that, I don't mind. My only criticism is that the colors (f/g) do not match a real 15C--The blue is a bit light and the yellow a bit dark--probably due to a delta in the designers monitor and the phone display. The key click volume also does not match the iPhone key click volume (it's too loud). When I need a 15C I use this one.
#2 has a lot of nice features. 4 level stack output. Horizontal and vertical displays. User definable colors and button sizes. All the Voyagers under a single icon. You can even move the stack from calc to calc (e.g. 15C to 12C). Macros. Issues: key lag and accuracy, annoying key click sound (also too loud, 41CX has a user definable volume for the key click--nice), for some reason settings are not being saved. Cheap, 4 calcs for < #1. I only use this for my 12C and 16C needs, but now that I have folders I may dump this for Tom's/Eric's 12C/16C.
#3 is just #2 with 15C only. Same issues IIRC.
#4 is like #1 but for $0.99. The rear view is poor. I do not recall why I deleted it (odd issues IIRC). 16Cx is not bad, but I do not like the display.
If you want a very solid, authentic 15C experience with a great rear reference, fast and accurate buttons, then get #1.
If you want a superset of the 15C then get 42s ($9.99) or Free42 (free), (please avoid RPN Calc 42s). I prefer 42s because is has a number of features making it really a 42S+ (e.g. larger stack display). Free42 is more in the spirit of Tom's/Eric's 15C--it's as close to a 42S as possible.
42s is my default everyday calculator, it loads fast, has nice complex math support (vs. 15C), the extra stack lines are great and can be expanded beyond 4 with 3 or 6 visible with just a swipe. The portrait view makes for simple one-handed computations.
Lastly, I must mention i41CX+ ($24.99). Of all the calcs listed above it's the only one that supports iOS multitasking (rapid switching to be accurate) besting 42s in load times. i41CX+ may become my default quick answer calc. The i41CX+ feature set it stunning. Not only does it emulate a loaded 41CX, but it also support all the modules including HEPAX and has great overlay support. It also has a printer, iOS cut/paste, email support, etc... See their website for all the details.
One more thing...
Not what you asked for, but if you are interested in 48/49 emulation then checkout m48+. Since it is based on EMU48 and not x48 like all the others you get a lot of the EMU48 features. And since there is a 15C emulator for the 48/49 that works in EMU48 then you could do that too. I do not recommend that, but I just needed a tie-in with your original request. :-)
NOTE: i41CX+ and m48+ are universal apps that look awesome on the iPad. The rest run in 2x mode--who doesn't want a giant 15C? :-)
REQUEST: Tom/Eric any chance for iOS 4 updates for fast task switch and perhaps a volume control for the key click? A Voyagers app with all 4 would be nice too--make it universal so that on the iPad I see all 4 on the screen at the same time with all functional of course so that when my 16C is busy computing ToH I can use the 12C to see if I am getting a good deal on my refi. :-)