I have both systems in my Lancair IVP (WSI on my Chelton and XM on a Garmin 396). The XM offers more features like cloud cover and cloud tops which is useful in IFR flying. Both NEXRAD pictures are very close. XM has a time loop feature which allows you to see movement of the precip. WSI has had issues with refresh rate not updating as often as it should and I hope this is addressed on their new version.

 

Light green shown on both systems may indicate precip (or not). Sometimes the screen will show it but you will not get any. I believe this is a function of NEXRAD antenna tilt and sensitivity. Carl's observation about icing and precip is correct. If you stay out of the green when you are in freezing conditions you can avoid the ice.  The XM system also has a screen that shows freezing levels and temps aloft which I have found to be pretty accurate and useful for staying out of ice.

 

As both systems are concerned -- if you are in above freezing conditions stay out of anything that is coded shades of yellow , red or purple. If you are in below freezing conditions with no ice protection systems stay out of all precip areas (green or above).

 

If you want to fly out of your coverage area, ie. Canada-- then contact WSI or XM and get that area added to your subscription.