Not even close.
GRT is only capable of presenting select portions of the wx data on the screen for you.
GRT Radar is decent, course the resolution is not good.
GRT winds is decent, but does not give you any additional features of the wind like best altitude based on current course and so forth.
Metars and Tafs work, but do not give you any enroute choices, only destination . You REALLY have to work to put in a metar that is not at a nearby airport.
Goo gobs of data are available to you in a well thought out format on the 496. Not much at all on the GRT. Clearly Garmin performed user studies and did a fine job of interactive presentation.
Going here
http://grtavionics.com/XM%20Weather.htm you can see all the TBD's. Its a long list of TBD items.
Good news is the cost of the XM addition to the GRT
IF you already own one is ~$1500. I would say its borderline worth it. I have it in mine and I am constantly frustrated with the lack of features and the usability of it. One flight with the 496 and the difference is staggering.
My 2 cents. I use the GRT XM wx and have for ~300 hours. What features they have do work. Some are difficult to use. None are broke. But when you take a high level look at all the 496 features and the GRT in the link above, its clear whats missing. An hour behind each will demonstrate huge usability differences.
Best,
