I've used both. If it were me I'd go UQ. So far the down UQ I've got just seems simpler, easier and possibly warmer. Smaller pack size and easier to handle than the foam layer of the HHSS.
The SS isn't a bad thing and I've not been cold with it but I prefer the down version, if only because I avoid having to squeeze the foam layer back into its bag.
Never been cold with either though so not faulting performance.