As probably one of the only people on here whose had 370s with both versions of the sat nav I'd say that how much it matters to you depends on personal preferences. I like listening to music in the car and I prefer to listen to my own. I don't recall the quality through the audio jack being bad personally but the more important point for me is that if you attach an MP3 player to an 09 car you can't change the tracks using the buttons on the nav unit itself - you actually have to do it on the MP3 player. With the Connect Premium you get track listings and album art on screen and you can control it via the unit - much better. If you're not so fussed about music, then it won't matter so much. Post code search is also a big help but you can live without it. I find that I don't use the touch screen that much because it bugs me to see it covered in finger prints (but that's just the OCD clean freak in me). Being able to fit a good reversing camera was a big plus for me though (you get one as standard in the Nismo thankfully).
On the other hand, everyone raves about uprevs but I was actually left wondering what all the fuss was about when I got my last 370 done - it felt only marginally different and I soon wasn't noticing any difference at all. Getting a car which already has a decent exhaust would be good though, as it's a big expense to sort out the weedy sound of a car with the stock exhaust.
So I guess it's a case of weighing up what matters to you most and making a choice. Personally, I'd go for the 2010 and then spend a bit more money adding the other bits because you can't do it the other way around.