Squid require cylindrical tanks, with NO corners. Cuttles on the other hand, usually only require a tank of 20-30 gallons for a single specimen. Cuttlefish (which are pretty hard to keep IMO) are damsels compared to squid, lol.
There are a couple of major differences between squid and cuttles, although they look similar. Cuttles (see Order Sepioidea on the Tree of Life, and Family Sepiidae) have a thick, calcareous cuttlebone inside their mantle, while squid (Order Teuthida, Suborders Myopsida and Oegopsida have a much thinner, flexible, chitinous gladius. Cuttles also have fins that extend along the entire length of their mantles, while most squid (but not all) have fins that are smaller and situated toward the posterior (pointy) end. Cuttles also have very stretchy, retractile tentacles, while those of squid are usually much less retractile (with a few exceptions).
Hope this helps.
A lot of cuttles have a more "w" shaped pupil, while squid's eyes are typically round. Do all cuttles share the 'w' pupil? Looking at Yahoo! Images there seems to be a varying degree of 'w'-ness, though this could also be due to light conditions. They'd be able to change their pupil size as humans do, right? Curious.