I have yet to see one. But you should also keep in mind comparison of professional services if you are developing and deploying JEE apps on a corporate scale (like we do); clustering and production/development support are critical. Training may be important as well. This is how these open-source companies make their money.
I am not going to make any comment for or against either framework (as I have no practical experience with Spring) although Spring is not based on a JSR whereas Seam and Guice are the basis for JSR299 (EE 6).
If you decide to go with Seam, will you use the entire Redhat/JBoss stack (including RHEL and JBoss AS and Richfaces and Hibernate and Seam, etc.)?
Will you purchase a subscription for the EAP 4.3 or 5 (4.3 includes JBosss 4.2.x and 5 will likely include JBoss 5) if your company purchases a subscription?
Most likely the Spring developer community is deeper and wider as Spring has been around longer than Seam (since 2003/2004 compared to 2006) and has a much higher adoption rate in the Java space. Most likely there are many more examples of external facing websites developed on the Spring/Hibernate stack than JBoss Seam stack.
If you contact Redhat sales and ask them for a list of companies using JBoss AS in production, they will not give you a list (trust me, I already tried). Not sure about Spring. In contrast, MyEclipseIDE has a list of their customers publicly available:
Do your homework!
From pressreleases one can obtain some information... http://customers.press.redhat.com/category/solutions/jboss/jboss-enterprise-application-platform/
Regarding Gartner studies on frameworks... now that will be the day...
The argument that Seam adheres to and integrates multiple JSR's (EJB, JSF, JPA etc.) instead of creating it's own is one of the stronger reasons we have at the moment. JSR-299 is definitely another thing we're taking into consideration.
Right now we do utilize Red Hat quite heavily in the server space, but I'm not in sales and haven't found anything regarding bundle pricing. The availability of EAP/JEMS support, if required, is another point that we see as advantageous though.
It would not be a full-blooded RH solution (likely not JBoss AS or Portal), but considerably more unified then it is currently; in terms of development technologies, it would likely use JBoss Tools and other Seam supported JBoss technologies(RichFaces, jBPM, Drools etc.) where possible.