I just had open more than 20 apps (iTunes, iMovie, iPhoto, Pages, Keynote, Safari and Chrome with a lot of tabs open, GameCentre...that kind of thing) and I was using 5.5Gb. So that is another 2.5Gb of RAM I could use before it would then throw out all App Cache (which was 1Gb), then that is another 1Gb before the system would then start to compress RAM, where I would then have 4Gb before I started to Swap heavily to HDD. That totals up to me using around 40% of RAM (with 100% representing the point in which Swap takes place). Seeing as I usually only have one Application open at a time, 8Gb of RAM is more than enough for me, and I plan to keep my iMac for at least 5 years before I would buy another one (and I would not sell it, I'd keep it as a second monitor, or even just sit it somewhere nice) so resale never meant nothing to me either.
I can't say to what would suit you better, but hopefully my decision will help you with your own
If your like me, 8Gb is great! If you are running VM's though, go for 16Gb.
I can't say to what would suit you better, but hopefully my decision will help you with your own