I think it help to distinguish between the two stages of songwriting...

First is actually writing the song. Having an arranger with a wide selection of inspiring styles is your main need here, and simple operation.

BUT... the next stage is making the demo (if you are making a demo to pitch to the biz). This is the area where deciding whether the arranger ALONE is going to be sufficient to make your song stand out from the pack. Knowing what your clients listen to from others, how 'slick' they want the demos, how finished they want the arrangement (be careful... often a fully realized arrangement gets in the way of appreciating a good song) is going to be probably where you decide whether to stick with the S910, and go the WS/software route, or whether blowing most of your budget on a better arranger is the best move.

TBH, there's a certain 'blandness' about arranger demos compared to demos played by live musicians or those done by pro keyboard players using WS's, at least at the upper end of the pitching scale. And, not to put Yamaha's down, but they come in at the slicker end of even the arranger sliding scale (that's a GOOD thing if you ARE going for 'slick'!). But only YOU know who your competition is, and what level they work at. It may be a PITA to learn how to effectively substitute WS or VSTi sounds for arranger Parts, but if that's what it takes, that's what it takes.

IMO, I would be wary of thinking that a better arranger is a magic bullet. As many (including Ian, for so long!) have pointed out, an S910 is a VERY close substitute for a Tyros. Personally, in your situation, I would perhaps look at blowing my budget on more MOTL arrangers from other manufacturers. Each one has something they do BEST. Each one has sounds that theirs are the best, each one has style areas they excel at. Plus, at the end of the day, you can hook them all up to your DAW on separate MIDI ports, and have a MUCH wider selection of sounds to substitute for whatever is weakest in the arranger you made the core demo on.

But knowing what and who you are up against is 90% of the battle. If you can't match them for production values, perhaps it is time to go the other way? Many a song has been sold with nothing more than an acoustic piano or guitar backing and one vocal. If it's a great song, it sounds stronger (and more versatile) like that than a fully realized demo...