Hi,
of course will testing another aid add a complexity to your decision process. But if you hear better in the end it´s worth it, isn´t it?
Here is a paper about the phonak fitting formula:
What a fitting formula is is described here (in german):
Siemens uses yet another fitting formula. What I was trying to say: If you use the same formula for both, they will sound almost identical. I had the impression that siemens with nal-nl2 sounds almost like phonak, and in the phonak paper they say that the phonak-formula is similar to nal-nl2 for moderate losses (which I have and you probably too). So before discarding the siemens, you could use a more agressive fitting. Also you should ask the fitting to be verified by real-ear-measurements (auf deutsch: In-Situ-Messung).
Last but not least: Make a speech test to compare both aids.
As for oticon: I have a bernafon (which is almost the same aid, different software), they will probably come with one button. Left aid: volume down, right aid: volume up. Medium long and long press can be assigned differently (program change for instance). The rocker of the siemens aid is better, but the oticon-button is still useful.