Few of the main difference between those tools are sizes and handle:
Farmer is a 93mm tool, with a little longer and beefier blade than the traditional 91mm SAK tool, and its handle is made of aluminium.
Fieldmaster is a 91mm tool, and it has the plastic handle.
Trekker non-serrated is a 111mm tool, and it also has a plastic handle. Some of the tools on this model is lockable, such as the main blade and the large flat head/bottle opener.
I think all of them are really good choices for outdoor use. If size is not a concern, then I would recommend the 111mm NS Trekker, since it has very relevant tools for outdoor use, and it's certainly one of my favorite folding knife/tool to take with.
Farmer and Fieldmaster are both nice, both are great for urban carry and especially good if you have the need to travel internationally(non-locking blade/sub 3" blade).