Google's marketplace is centered around Google Apps (email, documents, contacts, etc), while Intuit's is centered around their very popular accounting software. Both offer developers single sign-on mechanism, some access to their own applications for integration purposes, billing (actually Google doesn't yet, but will soon) . Both offer developers to host their applications and provide different levels of development tools. Intuit even goes as far as to offer file storage ala Amazon's S3, which Google doesn't offer yet.
One thing you notice if you look closer is that 3rd party SaaS apps are not yet deeply integrating with either service. Most implement single sign-on. Some will make it easy to import / export data to/from the app they are integrating with; but lacking the level of integration that would be considered significant. Perhaps this is just the beginning and the integration will get better from here; but something tells me that these apps will try to keep a neutral stance and avoid putting the hard work into deep integrations.
Bottom line? Integrated apps are better than non-integrated apps. :) You'll have to choose which to start with Intuit or Google Apps, and this can be a bit of a hassle..
All in all, I'm excited to be watching these developments as they can mean significant improvement to ease-of-use and productivity. Happy SaaS'ing!