It is rather difficult to answer this question without knowing what you mean by these two terms (both are used in a variety of contexts, and there is still debate about their precise definitions). The "low"/"high" level distinction identified by Izhaki is not universally accepted. It might be better if you were to explain why you think they are "pretty similar".
The most common understanding of "selective attention" is as a "filter", which prioritises the processing of some stimuli (e.g., a particular voice at a party, or a particular spatial location in a visual task). The wikipedia pages on attention have a reasonable summary of this. Importantly, we can selectively attend to something voluntarily (i.e., it is partly under conscious control), although in some circumstances our attention is also "captured" by distraction even when we don't want it to be.
"Sensory adaptation" can mean several things (for example the adaptation might refer to adaptation thanks to evolution, or to the way that our eyes grow accustomed to darkness). In current experimental psychology, the most common meaning of sensory (also neural or perceptual) adaptation is when the same, repeated stimulus evokes a different response. This may be because sensory neurons habituate, but it may also happen at the level of the cortex, and so in psychophysics it is a useful tool for learning about more complex representations. There is also a wiki article on this here.
One way in which adaptation and attention are similar is that both can modulate the response to particular stimuli (sometimes called changing the "gain" when talking about neurons). One way in which they are different is that selective attention is normally beneficial (it boosts processing), and requires conscious instructions or manipulations rather than simple repetition.