Well I personaly had the SQ5 and the SJ5 at the same time. The SQ5's sounded alot more muddy, meaning the bass sounded like it was bouncing arround in the headphone. They also let in more sound, my guess for what caused this would be the bigger driver housing. The big plastic square sides probibly weren't the best design choice for the headphones sound, but I personaly liked the design atleast ;P Although they looked tiny on my head, not as big as I thought they might be...
Otherwise the SQ5's were more comfortable and seemed to be built better, they were plasticy and by no means "Sturdy" feeling, but the headband didn't feel like it could break in my hands. They still sound good, aslong as you aren't criticaly comparing the two side by side like me you probibly wouldn't even notice this blurr in the bass. However its up to you weather or not you think the extra 50% in price is worth it. JVC has an Audio technica a700 clone that costs about as much (and in some cases less) as the SQ5's. So if you dont need the portability you might wanna look at those.