For an ultralight (or other airplane designed for slow flight), would a strut-braced or wire-braced wing be more efficient aerodynamically?
Depends on the shape. A round 1/8" wire would have way less drag than, say, a 1" round lift strut, obviously. And the same for a streamlined wire vs a streamlined lift strut. So looked at that way, the wires win aerodynamically even with 4 wires per wing vs 2 struts.
However, streamlined wires are crazy expensive and have to be custom made, so they aren't really a practical option and you are stuck with round cable, whereas a streamlined strut is not so hard to do.
So the practical comparison is round wire vs streamline strut. A teardrop shape has 1/12th the drag of a round shape, so a round shape of 1/8" diameter has the same drag as a 1.5" thick teardrop shape.
In view of that, a streamlined lift strut of say, 1" thickness (frontal area) would still have less drag than a 1/8" cable of the same length, and you only have 2 per wing vs 4 cables per wing, so overall you are ahead with streamlined struts. Add the benefits of rigidity, ease of assembly, looks, etc, and streamlined lift struts are a no brainer, which is why 99.9% of non-cantilever monoplanes use struts (wire bracing may be somewhat lighter however - or maybe not).
Now, a streamlined aluminum extrusion for an ultralight might also be somewhat expensive to source, though nowhere near as bad as streamlined cables. So for an ultralight, unless teardrop extrusion is available cheap, the most efficient all round choice would be to design a lift strut made from round aluminum tubing and apply over that a light fairing of .016" aluminum, or formable plastic, or fiberglass, or whatever, to get the teardrop shape.