I think that the sound quality of SACD vs. high res downloads depends more on the quality of your SACD player, your DAC, and the particular recording, rather than the inherent quality of the medium. I think that the decision as to whether to buy an SACD player depends on the type of music that you like. If you're a classical fan, there still seem to be more classical titles being released on SACD than as high res downloads. If you like classic jazz, and don't mind paying $30 per title, the SACDs from Analogue Productions are wonderful. But if your musical interests run more to rock and pop, then you probably won't find much on SACD to interest you. There are some older releases of classic rock that are still available, but those are getting scarce. Mobile Fidelity has been reissuing a few rock and pop titles on SACD, but it seems like there are more titles being released as high res downloads.
Another factor in your decision could be the type of playing experience you prefer. If you like easy access to your whole digital music collection, then high res downloads are best. If you prefer the experience of putting a physical piece of media into a player, then SACDs are better. (But not as good in that respect as LPs.)
