Both are only marginal at best, but I feel like my preference depends on my opponents. KTo is better for hitting something with the board (also susceptible to being dominated by something like AK, but naturally K5 is crushed by more potential hands), whereas I like 3-betting with K5s more due to slight
equity to the flush outs and the blockers (5 blocks the lower straights like 5-4-3-2-A and the King of course blocks against hands like AK).
I guess my answer is: "it depends"