Single bladed folder usually my Emerson A100, or a soddie/opinel/mini muscrat if in an office. And a tool box. SAK tools have never proven to be very useful to me, first because I have very short nails and just can't live with the nail nicks on an SAK, so for me leathermans are easier to use if I am going to be carrying a multitool type item, especially one with a changeable bit driver. The most used item on my multitools were the screw drivers, the leatherman bit set is darned near perfect except for the short/tiny shank on the thing, makes adjusting idle on carbs on the fly next to impossible, which is common for me. The Pliers on leathermans are also quite good, even compared to a number of decent sets I have, again, different peoples uses different suggestions, but for my life were its either fixing computers, outboard motors, or boats, SAKs have no place, at least with a leatherman I can use the bit in the plier jaws in a pinch.
A SAK soldier is is one that I did carry a few years ago and worked well at the time, but the tools still are just not there for me, YMMV of course.
As far as a single bladed folder vs an SAK, depends what your doing, for 90% of what 90% of people use a knife for the SAKs tools would be worth the extra bulk and less than ideal handle, with the one hand trekker being the best option with its one hand opening for convenience. But if you doing heavy cutting with your knife, for instance lots of work with rope, or lots of wood work, the ergonomics and tougher build of the modern single blade folder will definitely shine within a day of that type of use.