Personally I don't feel the tandem/side by side thing is much of an issue - the main advantage I see with side by side is that a considerate/attentive instructor can see you turning green (I prefer side by side if I have anyone else onboard). If you're willing to speak up that's less of an issue.
The CS prop does make life easier, and inverted fuel/oil is nice - I find stall turns neater in the decathlon, but it's not that likely you'll spend much time really needing it. I'm also a big fan of failwheels.. but you may well find you'll be able to fly the robin more often in comparison.
The short answer is there isn't a wrong answer
