These are quite different fluids, so you should first find out in your owners manual which one is recommended for that system.
The UDT is just a common universal tractor hydraulic fluid (THF) and if you look at most of these labels, you almost always find UDT listed as one that it "meets the specs."/suitable for, etc.. Here's one, the Shell Donax label that I found in 10 seconds on Google: http://www.canyonstateoil.com/msds-sheets/Shell_Oil/DonaxTD.pdf
The SUDT is a premium multi viscosity THF that is much more expensive, but it lasts longer and handles cold temps. better. Being in Georgia, if your manual recommends UDT, that should work well and be much cheaper. Personally, I would only use the SUDT if I was in a colder climate and/or the particular machine recommended only to use that one, or money was no object whatsoever.
Chevron makes a fluid that meets/exceeds SUDT specifications, but everywhere I read on the interweb says it costs even more.
Respected BITOG member Jim Allen said his testing of a particular "Farm Store" THF's showed it had a weak additive package. Everyone marches to a different drummer regarding what they think will work best in their machine.
Google will answer your questions.