I have a Series 3 alto and a Series 2 tenor, so it's not quite comparing like with like. However,
I would characterise them as follows: The series 3 is quite bright and "in your face". Would be perfect for funk or playing in a band with amps, guitars and drums. I have found I am looking for a slightly darker mouthpiece for this horn to reign it in a little.
Series 2: More "restrained" so to speak, but I find the sound a little more complex. Less brash, but almost a little "richer" if you can characterise the sound. However, you have to push a little harder on the series 2. Not an instant "In-your-face" effect like the series 3.
Also, palm keys are lower down and if you're used to them being in your hand, I now have to reach for the E palm key. I am finding it really rewarding to play the series 2 because the more I learn about breath control, diaphram support and airstream support, the better it sounds. Need quite a free-blowing mouthpiece though because it runs the risk of sounding a little muffled with the wrong setup.
Still fine-tuning both these horns and how to play them, but I really didn't like the series 3 tenors I playtested. Series 2 altos were quite similar in characteristics to the tenor, as described above.