If SCSI DVD drives were hypothetically bootable on system 7 and/or system 7 had a UDF driver or we could make HFS/HFS+ formatted DVDs -- we'd still need to balance between DVD and MO in terms of what type of longevity we're looking for. The same is true of CD-ROMs and CD-ROMs have the advantage of that there are a lot more of 'em hanging around.
I've casually looked this up, and it seems that DVD UDF support wasn't present until Mac OS 8.1 or higher. Whether it could be made to work on earlier Mac versions is up to whether anyone here has a SCSI DVD drive that supports Mac OS and if they can test on a 68k Mac under 7.5.5 (the most backwards-compatible Mac OS).
Still, I think there are advantages to magneto optical over DVD, including DVD-RAM:
1. They can be modified on your classic Mac machine. Mounted RW disks can be written to. This is helpful if you want to boot from one.
2. You can boot from them. Not sure about booting from DVD on a 68K under 7.5.5, I have a feeling it would never work.
3. Availability of drives and media is still somewhat plentiful, at around the $100 mark for drives and around $5/disk for media if sourced from the lower priced sellers. I got some 2.6GB media at under $2 a disk (new sealed), and just picked up some more 9.1Gb disks for under $4 each (used).
4. A SCSI disk driver works on MO disks, meaning the OS sees them as any other SCSI device (at least for 5.25" 9.1GB drives I've tested). On 3.5" the MO Drive Extension linked to earlier in this thread works on all of the drives I've tested, from 128MB through 2.3GB on Mac system 7-Mac OS 9.2.2.
5. Longevity - I believe MO disks will outlive DVD (either DVD-R or DVD-RAM phase-change) because of the nature of the recording medium. Dye recordables are questionable after even just 10 years in some instances.
6. They're cool! You can custom print labels on fancy color printers and apply them to disks, and they take on an 'official' look. Using my MO disks with fancy labels feels like using an official Apple product, and hand-marked CDs and DVDs just don't feel the same. Maybe it's just me, liking to be 'fancy'.
7. The 3.5" drives are easily retrofitted to mount inside any 3.5" slot, and can easily be used behind a Zip drive bezel, and even the 3.5" floppy drive slot/bezel. Who needs a 3.5" floppy anymore anyways? Put a 1.3Gb MO drive inside an LC475 and you have reliable, fast, cheap storage accessible to you. Add a USB, FireWire, IDE, or (with SCSI card) a SCSI MO drive to your more modern Mac, and you have a very good bridge that is also vintage.
As you can tell, I'm a fan of retro/vintage solutions. I don't use any SD card solutions in any of my Macs. I do have some to try out in the future, but for now I've been more than satisfied with how these vintage solutions work.