I own new Stepwgn before, then I sold it and bought Serena, I personally like Serena more coz' of :
Serena 2.0
- built quality (you will find out when you have car wash)
- more powerful to go uphill
- much more comfortable on 3rd row seat
- dual sunroofs and ion air
- seat arrangement
- quiet on road drive
- driver seat height adjustment, 05' Stepwgn doesn't have it, not sure now
- 2008 Serena has HID/power trunk/better look dashboard/food tray
Stepwgn 2.0
- dashboard look
- outlook (a bit better, but Serena can also have spoiler set installed too )
- handling