Out of the three alternatives I would go with the first too. I think OSGI is more relevant for modularising at infrastructure level - not at application level.
Thinking outside the given alternatives, I would actually just go with a Spring web application rather than Grails, without an extra layer interacting using webservices. I would of course still separate the business logic into separate Spring beans, so that if load-balancing is needed later on, you can separate out the business logic beans in to multiple back-end servers, but the presentation layer can still interact with the logic using the same interfaces (using Spring remoting, Hessian/Burlap or webservices). Also, other presentation layers (Swing, mobile, etc) can interact with the same business logic beans. Otherwise I think you will end up with a complicated architecture even if you don't ever need load-balancing in the future. I suppose it comes down to whether Grails is your preferred presentation layer.
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