I home demo'ed Qsonix for awhile in my system but never had a chance to try Sooloos. I ended up not buying Qsonix for a few reaasons.
Features that I found very important that does not support by Qsonix (as of 4-5 months ago, not sure about now but may be worth checking out) are:
1) lack of gapless play. I have lots of operas that require gapless play ie no pause between tracks. Qsonix does not suppor this so there is always a little fade in and out between tracks.
2) When using Qsonix to rip an album, it does not let me merge multi CD Boxsets into single album so each CD will be its own album . Again, I have lots of operas, classical music etc, probably close to a third of my CDs are boxset. The only way to go around that is to rip these CDs with my computer and import these albums into Qsonix later which was a bit of a pain.
3) Sound quality wise, the new Qsonix/Wadia with its digital output into my Playback MPS-5 via XLR is very mellow and laidback, nice for certain music but as a whole, I prefer my own computer server/XXHighend/Weiss INT202 firewire/XLR converter. Although I have to say that QSonix is much more convenient to use than my computer server. But at least, at the time, I did not feel that it was worth it to replace my computer with Qsonix just for the interface, considering the cost factor.
4) Searching through music library is still a pain. I think it is well setup for rock, pop etc but for classical music, browsing first through genre then composer etc is not possible. Here I find the flexibility in indexing and browsing capability is still a bit limiting in comparison to say iTune or MediaMonkey on computer.
I talked to Qsonix quite a bit during that time, some features they are working on but gapless play and merging album is way way down their list of things to do and they may not get to it at all.
The best thing to do is try out both Sooloos and Qsonix interface for awhile until you really know their capability and limitation. Qsonix unfortunately did not quite work for me. However, your requirement may be different.