Streaming Data and Why it's Important
Before I go on talking about Buffer size and EIDE vs SCSI, It is important to know a little about how CD-RWs write data. The key point here is that the data must be streaming, and consistent, because the laser writes regardless of whether or not it has anything to etch in or not. This is one of the reasons why there are very few CD-RW drives which surpass 4x CD-R(W) writes. It is difficult to guarantee a consistent 4x input for the laser.
In order to help ensure streaming uninterrupted data, CD-RWs make use of "on-drive" memory as a buffer. The system hard drive (or any other input source) sends the information to the CD-RWs buffer. In turn, the CD-RW then reads the data from the buffer (since the buffer is RAM, it is very very fast) and burns the data into the CD. The size of the buffer is very important to help maintain consistency and allow the user to run other programs while burning a CD.
Why is the size so important? Well, let's say you want to burn a CD and play Quake2 at the same time (not recommended) While Quake2 is loading, the CD-RW software will be accessing the HD for data to burn. If the drive is too slow to the supply the CD-RW drive (actually the buffer) with data fast enough before the data in the buffer drains out, there will be a Buffer Underrun error and recording will be stopped. (Since the laser head cant move backwards to go fix gaps) For this reason, a large buffer is very helpful, since it takes longer to "drain out".
EIDE vs SCSI
The important difference between EIDE and SCSI as it pertains to CD-RW drives is CPU utilization. A SCSI CD-RW drive is controlled by the SCSI controller (a processor), which means that there is virtually no CPU utilization during record time. This is very helpful on (a) slow computers which may have trouble keeping up, and (b) multitasking.
EIDE, on the other hand, requires the CPU to aid in the recording process. If the CPU is doing any other tasks, there is a chance that it won't be able to keep up, resulting in a Buffer Underrun error. This will ruin the CD-R(W) disk.