Far larger than Don Dhet, Don Khon is skipped by many budget travellers because most of the accommodation is mid range. However although there isn't 40 odd places to choose from, there are budget options here and staying on Don Khon is far more of a Lao experience than Don Dhet.
There is a better range of eateries than on Don Dhet and the options for cycling and walking are considerably more extensive. The main disadvantage (aside from the generally higher cost of accommodation) is that in early to mid afternoon the island is swarmed by day trippers from Pakse and Thailand who plumment onto the island in droves -- at least they are only there for an hour or so.
The island has a few old colonial buildings, including the school, the French port and the hospital (now part of a resort) along with the French embankment -- a series of concrete barriers built on the eastern side of the island, remnants of the train track (mainly now just the ballast and a few sleepers), the waterfalls at the eastern end of the island, the dolphins (if you are lucky) off the southern coast of the island and pleasnt village life that you can cycle or wander through at your leisure.