I'd expect both versions have 24bit converters. Simply because that's the norm for those parts. They can run with less bits no problem. The difference is the SX takes 16bits straight while Og compresses from the incoming 24bit. But the compressed data while stored at 16bit throws some data away and rebuilds it on playback. Compression's about 50% yet does sound pretty good. But the errors are enough that they get bigger if you re-record (or re-sample). The effects of the compression have been fairly well investigated by Roland/Boss digital recorder users and even the latest Boss 8track recorder still uses it.
That said, even 16bit uncompressed starts to deteriorate if you keep re-recording it, but you can do it many more times.

Jim

On the SX I tested exactly that: I resampled the same sample over and over.
After 10 times there was a little bit of noise added, after 20 times it became more noticable - but there was no noticable degradation in sound otherwise.
Conclusion: it's safe to say that under practical conditions, there is no degradation in sound while resampling on the 404SX.

This amazing level of resampling potential on the SX has left me completely baffled as to why there are so many people left debating the merits of the SX against the regular 404. They are only kidding themselves. This thing is a monster!

Roland, for being mostly jerks @ NAMM and in customer service, are at least fairly forward-thinking in the competitive nature of their designs. Akai screwed up by not having ambiguous patents like Roland...Roland makes it impossible for other companies to make products like electronic drum kits, due to the vague patent they have on "round electronic surfaces intended to be struck with a stick." If Akai had pattented square rubber buttons...but I digress...

Get the SX, if you can afford it. The cards go up to 32 gigs, so you can do countless backups even with the uncompressed waves.