I have had both and, much to my dismay, very slightly prefer the Portas.
I really do not like the more elaborate pad adjustment on the Portas, but there is something about the suspension of the plastic earcups that support the drivers on the Sporta that seems to blur the sound (mostly in the bass region) a bit for me. I tried 3 different Sportas and they all sounded the same.
I beleive there is a review on Goodcans.com that references this:
http://www.goodcans.com/HeadphoneReviews/ipod.htm
Koss Portapro - For well over a decade our favorite headphone for sound quality under $50 has been this headphone meant for portable use and thus good for connecting to most portable players. The Koss Portapro, (not to be confused with the Sportapro which doesn't sound quite as good), offers good detail in all areas; highs, mids and bass. In addition to great sound quality for a portable headphone, it’s light in weight, comfortable, and collapsible. We've sold and listened to this headphone since the late 80's... it's got a long proven track record! Note: Sennheiser has more recently come out with a competing, foldable portable, the PX100. Actually, it's a good headphone and if the Koss PortaPro didn't exist, we'd recommend it. But, we do still like the PortaPro slightly more than the PX-100.
Also refered to here:
Sportapro Sound Quality - When I first reviewed the Sportapro, I was excited that Koss had take the Portapro and gave it such a versatile headband... and at a cheaper price! While we have had very many customers pleased with their Sportapro’s, one customer with a very discerning ear mentioned a minor problem he sometimes heard with the bass. I tried his pair and occasionally with very bass heavy music, (something I do not listen to all that often), I could hear a fluttering sound. Trying to recreate this phenomenon, I discovered that it only seemed to happen when that bass heavy music is played at louder volumes, just a bit louder than I normally listen to my music, (remember, protect your hearing and don't play your headphones too loud). Curious, I took a closer listen and compared a Sportapro with a Portapro. Guess what? I couldn't hear the fluttering with the Portapro's and in fact, the sound quality of the Portapro’s was constantly better when listened to closely!
Comparing the specs for the Portapro's vs. the Sportapro's, the only difference seems minor? The Sensitivity changes from 101 to 103 dB SPL/1mW on the Sportapro. I had noticed this difference before and I assumed a mute button on the Sportapro was slightly affecting the numbers, (but now the Sportapro no longer has that mute button and the numbers are still the same). Looking closely at the transducers, they do appear to be the same. But here is a lesson I have learned about the effect the housing has on headphone sound quality. The Sportapro has some additional plastic behind the transducer and it appears the sound is bouncing off of the plastic and having a negative effect on the sound. For active use, we still recommend the Sportapro as it still has better sound than most portable headphones! When you are exercising or walking, most people don’t listen closely enough to even hear the slight sound degradation. But for the best portable sound quality in non-active use, we do prefer the Portapro!
and I have never read anyone else reference this difference other than they slightly prefer the sound of the Porta.