There's a few theoretical differences, things like crimp in the weave etc. Theoretically, a lightweight woven cloth will be stronger than a heavyweight woven cloth because as you say, there is less crimp in the weave and hence less of a tendency for the fibres to want to straighten out under load. In the real world, how much difference this actually makes is going to depend on the application. For most applications, the economic benefit of heavier weight materials being cheaper and taking less time to layup is more of an influence than the theoretical difference in strength.
The other factor of lighter fabrics is you have more opportunity for fibre orientation for a given thickness, and they are easier to manipulate around geometry. For flat sheet, this obviously isn't normally too much of a concern, and so heavier weight materials are an advantage. You'll also find that especially with carbon, one layer of 800g biaxial infuses easier than 2 layers of 400g biaxial.
All in all, I think the heavier fabrics are very suited to your application - you wont get much benefit out of using lighter weight fabrics.