Hey! Always good to see another Amiga user in the USA...so many of the folks here seem to be across the pond, so it's nice to have a little balance
amiga.resource.cx is your best friend when it comes to upgrade information. In this case, it looks like the primary differences between the 2090 and the 2091 are that the 2090 also supports Shugart-standard hard disks (not very useful in this day and age, but there you go,) but does not support booting from the hard disk (very
not useful.) The 2091 can auto-boot, and also supports XT-IDE disks, for what that's worth (another format nobody uses anymore except us vintage computer enthusiasts.)
The 2091 is most likely your better option, but I do have a
GVP Impact SCSI controller that you can have for the cost of shipping. It's nothing fancy, but it does work and supports auto-boot. (As for software, auto-boot requires Kickstart 1.3 or newer - I have a spare KS1.3 ROM too, I believe.)