Hi, sorry I just saw your post. You may have already decided for your son on which appliance to use, but I hope this is good information for everyone else.
As far as the effectiveness in widening the arch, current research sees both slow expansion and rapid expansion achieving similar results with insignificant differences, except a slight more dental side effect with slow expansion. Stability is also the same for both treatments. Both treatments will get sutural separation on the palate on growing patients. (see links below on a couple of articles. There are a lot of studies done online). They are similar because once the palate is expanded, you need to hold it for 4-6 months for the palate to stabilize. During this time, the midline area will remodel and stabilize in the case of rapid expansion, and teeth will upright in slow expansion. Quadhelix is tricky to adjust though, I personally prefer a different type of appliance if I'm going to do slow expansion.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4138550/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4347413/
From a comfort stand point, slow expansion is more gradual and patient will feel less tightness, but it's a bulkier in the mouth.For rapid expansion, it's fairly uncomfortable in the first week because of the tightness (I recommend some pain killers), but once the suture is loose after the first week, I don't ever hear patients complain about the tightness.