There is no point to having both IMO. So if you really want a new keyboard you could just trade your Tyros 3 in at any major music retailer.
The touch screen on the PSR-SX might make things a little easier. But the operating systems and features are almost the same. If you understand the Tyros' "navigation metaphor," it's not really any harder to learn and use.
Everything on the screen is layed out in approximate rows and columns. The buttons on either side of the tilt screen let you choose the row you want to work with. The rocker switches below the screen (especially the "up" button) lets you choose your column. The little pair of switches above and to the right of the screen let you scroll back and forth through screen "tabs."
There's also a handy "direct access" button which works like a "Shift" key. If you press Direct Access and then ACMP, it warps you to the Accompaniment settings.
Once you have a screen field selected, use the data wheel or +/- keys to change the value, and then press Enter.
Don't be afraid to experiment! If you really screw things up there's even a factory reset. It's in your manual but for many Yamaha's you just turn the board on while holding the highest key.
I also think there are commercial DVDs available that explain the features of your keyboard.
For more support info, go here:http://www.psrtutorial.com