The MSRP on the receiver is $1000 (which will translate to about the same as the XM data receiver), but the site suggests this will only work with the proprietary multi-function displays shown, which themselves are retailing for $1,500-$3,000. There is no mention of a data interface for a PC.
So, your initial investment for the Siruis system (receiver and display), if I'm reading their info correctly, would be from $2,500 to over $4,000!
The data subscription plans look to be almost identical to XM, although with Sirius you get lightning data with the cheapest plan ($30/month).
Possibly the biggest problem is the data that I don't see listed as included - Sirius doesn't appear to offer any county warning data or Baron-like shear product, and it's not clear whether the Sirius storm tracks would include meso (rotation) markers.
If I'm not mistaken, unless 1.) their receiver actually does work with a PC and 2.) there is actually some sort of shear/velocity/rotation product and warning data, it looks like the Sirius system is not an option for chasing. It seems from the web info that the Sirius system is specialized for marine use only, and not land based or vehicle-based use.