Well, the MSI HSF is bigger and looks to have a bigger fan, which bodes well for it being the best in terms of cooling and noise. I'm pretty sure a few people here own one, given the many recent threads about this card, so maybe they'll stop by and post their experiences.
The MSI also seems virtually guaranteed to have a R360 (9800XT) core, which probably means better OCing than a regular R350 core (9800P) simply b/c the R360 is made to operate at 412MHz minimum. Still, I'm guessing more and more 9800Ps will ship with R360 cores, so the Sapphire card may well have it, too (SupremeCommando's 7/16/2004 1:51:47 AM NewEgg review indicates otherwise, though his core OC was still decent).
I don't know anything about the memory on either board.
That leaves warranty. If you buy OEM from NewEgg, the warranty should be the same either way.
In terms of build quality (HSF and board itself), you'll have to ask actual owners. The MSI may use cheaper parts: apparently its capacitors are cheaper and less tolerant to high humidity than the all-metal ones on the Sapphire board, and the MSI board itself looks like a significant redesign from the original layout (probably for cost reasons).
russr, I'd think the best recommendations would come from owners, no? You won't know how well the card OCs or how loud the HSF is unless you own it. Otherwise it's just talk, like my post. 🙂