Having a stylus does not enable a straight trade for a DSi. The DSi (2009) is a newer, upgraded model; the original DS (2004) / DS Lite (2006) are older and less valuable.
Stylus is a $1 accessory included with every unit—it adds no negotiating leverage.
DSi adds cameras, SD slot, DSiWare, better screens, no GBA slot. Decide if those features worth the cost.