well if it was me id go for just the dvd rom,same as you already have a cdrw drive it wouldnt make much sense to buy a combi,i dont no if there any less reliable but im sure someone else round here could!
Stick to a DVD-ROM drive - apart from the fact that it's dedicated to its purpose of reading DVD (and CD)-ROMs, multi-purpose drives such as rewriters and combos do not enjoy the same reliability and longevity as CD and DVD-ROM drives.
Manufacturers' figures prove these facts and are readily available in website specifications for the various types of drives.
I've had two rewriters (both by Plextor and rebadged as TDK models) fail within 12 months, yet basic CD-ROM drives regularly updated as speeds increased, are still going strong in friends' systems.
In turn the CD-ROM has been replaced by a Pioneer 116 DSVD-ROM drive, still functioning splendidly after two years' use.
I bought a Branded Combo drive 2 yrs ago its had a heavy work load And has given no trouble what so ever. I prefer the combo as It leaves me more room for drive caddies.My only detrimental comment about my drive was it was expensive £186:23p inclusive..But it is down to preference and usage planned vs depth of wallet/purse