I have sailed both but do not know which is more efficient. I suspect a sloop rig is because you do not see high preformance racing boats with gaffs. For day sailing on a home built boat on the lake it probably does not make a meaningfull difference. One advantage of a gaff on a small boat is the length of the spars. Instead of one long pole that can make trailering and storing a problem, you have 2 shorter spars. If set up properly, raising the shorter gaff mast might be easyer too.
There are sloop options. West wright potters are one example.
I have been looking at CLC boats pocket ship kit recently (this site will not let me post the link). I have no experience with the boat itself, but have built 2 of CLC's kayaks and found the kits to be well done and the product support to be great. It looks very similar to what you have in mind.
You can add more lines and a spinacker to almost anything, not that it always make it a better boat.