Solo backpack sprayer - Piston or Diaphragm?

I recently picked up a piston one. I seemed to work fine when I put down MSM and Prodiamine.

I have a piston in my 4 gal. backpack sprayer.
Just looking online there are some differences.
The piston can get damaged more esaily with abrasvie additives then the diaphragm pump, I haven't run into that problem yet and I spray all of what you listed plus some, MSM, Celcius, Atrazine, 24D, clorox, suspend sc, simazine, propiconazole, bifenthrin, etc. This will be my 3rd season with it. I rinse it, spray atleast 1 gal. of clean water through it, release pressure and take nozles off before storage.

Sounds like your next step is to get an idea of your lawns square footage and then an idea of your flow rate with the new sprayer.

I spray ~2000 SF with 4 gallons (full tank) using the little red fan nozzle my Chapin sprayer came with.

the 4 gallon one from Harbor Freight has worked just fine for me. It's $20. I use it mostly for spraying Talstar and Eraser. I figured it would break after a few uses, but i've used it at least 20-30 times over the last few years, and it's still working just fine.

You aren't going to notice that much of a difference between the two. I find the piston a little easier to work on if they have issues. Your solo backpack sprayer sprays at .67 gallons per minute when you use the fan tip sprayer (you should always use the fan tip)