8MB cache does seem silly but it theoretically allows for continued burning while lesser models have to pause as buffer underrun protection kicks in. In the real world it probably makes little difference but then neither do slightly incremental burning speeds. FYI, only the retail models outside Asia (read: North America, at least) have 8MB cache. The VAR/OEM have 2MB. Also, the 48X may require a ROM update for Mt. Rainier support. Both the Samsung and Hitachi-LG are available from Dell (as potluck Dell brands) so I would be confident in either reliability-wise. Samsung certainly seem dedicated to support and innovation if you check out their website. On the other hand, HLG may be better on the quality side. It is hard to judge because people usually only post when they have problems. Anyhoo, check out cdfreaks, cdrlabs, and rpc1.org for more info.
P.S. the Samsung has a RPC1 hacked ROM available, whereas the HLG does not (yet).