Have both also
my husband prefers the Tempest 170 for its cockpit fit mainly - the sirocco is pretty large in the cockpit and although he weighs around 225 its outfitting doesnt fit him as well. He also likes his sons P&H Capella 166 - a larger fit for his size. Good luck!
not so much length but rocker
– Last Updated: May-27-09 8:23 PM EST –
that determines the tracking characteristics of a kayak---the more pronounced the rocker on a kayak the more manuverability the boat has---the straighter the hull the more tracking ability the boat has--a skeg on a boat with more rocker will help the tracking ability--My Tempest 170 has quite a bit of rocker but the the skeg deployed it tracks fine.
Sirocco vs. Tempest
For what it’s worth, my 2 cents.
The Sirocco
Shorter, more rocker, wider, more maneuverable, not so fast. Poorer at tracking, especially in a following sea. A rounded hull = steady, mushy ride. With more volume, able to carry a bigger payload (230 lbs of me). Good bungee layout, but lacking the outfitting for ugly tubes/paddle holder up front. Back deck higher than you might want for rolling but okay. Better hatch covers than the tupperware Tempest. Large cockpit with poor thigh braces. Intangibles: better looking (subjective). Can be found for cheap used, but not as easily as a used Tempest. Plus or minus, you decide: artwork on the bow.
The Tempest
Longer, narrower, better tracking, but still highly maneuverable and fast. Great outfitting in the cockpit if you don’t mind straps here and there. Soft chines and boxy. Excellent thigh braces. Rolls like a charm. The best bungee outfitting of any boat. Problematic hatch covers on the plastic boat; people have complained about leaks. Can be found for cheap used, easier to find used than a Sirocco. Plus or minus, you decide: no artwork on the bow.
And unlike everyone else I say buy a boat and paddle it. If you don’t like it, trade it or sell it for something else. 
Happy paddling! Cheers.
Consider Necky too
The Tempest 170, Scirocco, Aquanaut, and Chatham 17 were on my buying list and I ended up with a plastic Chatham. Longer / faster, with less rocker than the Chatham 16, while very maneuverable on edge and freakishly stable for a 21" wide boat. It is however lower volume than “low volume” boats, which helps with weathercocking, but hard to pack for a week long trip if you aren’t a back packer. You really need to rent several boats and put each one through the paces…
weathercocking
it’s the hull shape and not just the windage,low freeboard, that makes it so managable in high winds. The Chathams low weathercocking is one of the main reasons I’d pick it over the Tempest or Scirocco. The Tempest weathercocks moderately but the skeg trims it fine,if your skeg breaks you’re pretty much screwed in high winds. With the Chatham if the skeg becomes non-deployable you’ll have a lot easier time staying on course than the Tempest.
Same problem with the Scirocco. I really wonder if they tested it after they built the mold because the skeg doesn’t correct for weathercocking that well. It feels like the pivot point is too far forward whereas the pivot point in the Tempest is in a good spot, it simply weathercocks a moderate amount.