This is a little bit disorganized because it's early, but:
As far as I know, the SCSI2SD v6 is still the drag race champion among the modern solutions. Mine gets ~7MB/sec read and ~4MB/sec write in my 8600/300. My benches are over at
https://68kmla.org/bb/index.php?threads/cheap-ide-on-scsi-bus-solution.32521/ - in that thread we found that using a high grade SD card does matter.
The numbers Paulie posted for the ZuluSCSI look great and I'm looking forward to getting one or two for some of my machines that need modern storage. I have a SCSI2SD v5 in my 840av and likely don' want to pull that machine apart again, but my personal theory is that ZuluSCSI is gonna be Good Enough for basically all my other SCSI-havin' macs. Realistically, even my beige G3, unless I wanted to get serious about video capture with that machine, for some reason.
I know this isn't what was asked but I think what's "important to get" really depends on the type of build you're looking to do, what you want to do with it, what you want to get out of it, etc etc.
Classic Mac OS is terrible at all i/o, networking and disk especially. It makes up for that by being very lightweight, even for its time. You simply don't really need good disk performance unless you want to do video editing or similar multimedia authoring or win at benchmarks. To add to cheesestraw's point -- seeks are more important to how fast a computer feels -- even on Classic Mac OS, which, again, mostly doesn't do a good job taking advantage of more throughput.
(Basically -- I'm perhaps not really getting much out of my SCSI2SDv6 winning drag races in my 8600, especially what with the way people talked about getting like 2MB/sec on a 9500 being impressive -- it's not
impressive but it's probably perfectly fine for most things.)
With that in mind, another option to win benchmarks (and capacity and reliability) as you get to the PCI era is to drop in a SATA card. Should outdo basically everything else in practical terms, because you'll get the high speed of SATA itself as well as the low seeks of SSDs. The only real bummer is Classic Mac OS has a 2TB volume limit.