Two totally different jacks for a wide range of interest.
FARM JACK Can't go wrong with an actual name brand Hi-Lift. If you want to save money Reese is crap in my opinion. The forged (cast?) Harbor Freight Jacks are better. I don't like the stamped and welded from any manufacturer or reseller.
MACHINIST JACK I haven't used one. I do almost exclusively CNC and I want my machining cutting the part as fast as possible. Time IS money. I've always been afraid that a jack may slip if the machine is doing hard fast reversals. Is pinching it between the piece of stock and the table enough to keep it from moving? I do a lot of aluminum so flex is really an issue. Most times if I need to cut larger pieces of stock I usually put two vises on the table. Sometimes its not practical or extremely inconvenient. For example if I have a setup on part of the table I don't want to tear down because I'm half way through a multiple piece order, or on the smaller machines if its just not practical to have two vises on the table. Do any/many of you use a stud on the bottom of a machinist jack to secure it to the table with a t-nut? What about a swivel pad on a jack? Is that a good idea?