[Galaxy S I9000 General] bitbang3r:
Obviously class 10 is better if cost is literally no object... but if you're going to go with class 10, take the time to really, really research the benchmarks before buying because they vary wildly between brands... and aren't entirely consistent within brands/models, either. It's not entirely a question of quality, either. Basically, class 10 cards fall into three observed categories:

a) Cards that are 100% optimized for brute-force write speed of large, sequential blocks of data and long-term reliability/data integrity. These cards tend to suck miserably in benchmarks that concentrate on small, random-access reads & writes, but make the public relations department happy because their bulk sequential speeds look positively *unreal*. Sandisk is the almost unquestioned leader in this category.

b) Cards that do everything possible to make small random-access reads/writes as fast as possible. These cards are roughly 50% faster than category 'a' cards in benchmarks testing small random-access read/write activity, but only have about 90% of the brute-force performance of category 'a' cards for those large sequential reads/writes. These cards are about double the speed of a random, non-trash class 6 card... at roughly 4 times the price. Oddly enough, from what I've seen, Sandisk cards do NOT fall into this category. Rather, they tend to be Taiwanese brands sold to enthusiasts by companies like Newegg.

c) Cards that fall somewhere in between, but mostly give you the worst of both.

For Android swapfile use, you want a card that's as close to an ideal 'b' card as possible. Bulk read/write speed really doesn't matter unless you're using the card for HD video capture or rapid-fire mega-megapixel action shots. Random-access speed matters ENORMOUSLY.

As for the Sandisk anomaly, I think it's because Sandisk designs and markets its best cards to and for video professionals who literally care about nothing besides bulk video read/write performance (with long-term stability & reliability a close second), and for whom cost is literally no object while the other high-performing companies fight over the table scraps provided by the "enthusiast" market who use microSD cards in ways they were never really intended to be used (they were NEVER intended to be high-performance storage for small random-access data use). The smaller companies basically cut corners on long-term storage life & overclock the controllers, because they know that 99% of the cards will be junked by their owners in 2-3 years for being too small or too slow relative to the newest cards, while Sandisk's market might very well expect data stored on a card to still be error-free after spending 10+ years in a drawer.

That said, if you're going to go buy a random stick of flash more or less sight unseen, your best bet is class 6 just because it doesn't have quite as much variation between the best and worst, and it's cheap enough if you stick with 4-8 gigs to not leave you regretting a mistake for the next year. From what I've read, the best class 10's random-access performance is about twice as fast as a random stick of mediocre class 6. However, the most expensive Sandisk Pro class 10 is only about 50% faster.

At the end of the day, there's no known performance consequence to going with class 10 if you literally don't care how much it costs... the worst-performing class 10 is still going to be at least as fast as the best class 6. Just a lot more expensive.

Another thing -- don't buy more than 4 gigs unless you really NEED to. Android is kind of like Windows (or at least it is under SenseUI, and probably TouchWiz too)... it feels compelled to touch your files over and over to reassure itself that they're still there. Take two identically-flashed Android phones, but stick a 4-gig mostly empty card into one, and a mp3-packed 16-gig card into the other. In most scenarios, the one with the packed 16-gig card WILL be observably slower in daily use.