to prettyinpink we stayed at the palace and the sahara beach last year within 8 weeks of each other (sept and nov) the food is much better at the palace the gardens and pools, even if the tropical is closed are far superior to the sahara beach, although the beach in tunisia is better and cleaner than the palace beach.
we walked to sarigerme quite a few times took us about 15 minutes,and sarigerme is a quaint village with shops and a market on sunday.
Sousse is a god forsaken place that once visited is not a place you would go back to, unless you like mixing with thieves con men beggars short changing crooked shopkeepers, small ,(and i mean small) children constantly harrasing you for money. it is a country mile away from the sahara, we tried waking it, after about an hour we finished the journey on a bus.
if anything though,the palace,in spite of sarigerme being close, is far more isolated than the sahara, although i think the sahara is more dangerous, the only reason you wouldnt get mugged outside the palace is that even the locals wouldnt go there at night.
personally if i had kids i would go back to the pegasus, but they havnt made enough wild horses that would get me within spitting distance of that awful hotel in tunisia