I agree 1GB for apps is plenty, I have 1.8GB on my Samsung and despite having something like 140apps now (including a lot of high quality games) I haven't run into limits yet.
A lot of the commentary here regards OS updates on Samsung handsets isn't particularly well informed I'm afraid. Samsung deserved a bad name for after sales support for handsets they had released prior to the SGS, but I don't really think thats a particularly valid criticism right now. I think the prolific amount of bad press about slow updates in the US is being taken out of context. In that market the delays are more the result of the carriers, rather than Samsung. Outside that Market the SGS has been fantastically supported. I had Froyo on mine in August unofficially, and the official release came Sept-Oct from memory. While that is later than the Desire, if you look at it in relation to their relative release dates given the fact that the SGS became available outside Korea in something like late May/early June, the update timeframe is essentially perfectly commensurate with HTCs Froyo update for the Desire. Now with the Gingerbread update the SGS will get it officially before the Desire. Two timely OS updates with so far nearly a year of after-sales support is nothing to be sneered at in the Android realm, and equal to HTCs support of the Desire.
I know that Cowon S9 has a great reputation for sound, I'd be interested to know how it compares to the SGSI (particularly with Voodoo sound), most of the commentary I've seen from audiophiles rates the SGS extremely highly. Sound quality in my phones is extremely valuable to me, since I use it as my primary portable music player. I'm not an audiophile by any means, but I would find it hard to swallow going backwards with regards to audio quality, and the SGSI has easily the best sound I've yet encountered in a phone. Based on past history sound quality is likely to favor the SGSII over the Sensation, so for me thats an important differentiator. For Kopkiwi it isn't, and I totally respect that. As I said before, they are both fine phones, which is best is up to the individuals use-case scenarios. As with speed of operation this still remains to be seen, if it favors the Sensation then I'll to look again at which phone compromises the things that are important to me the least.