As with anything it appears there are a range of companies. We found that many places which deal in both manufactured and modular homes seemed to be the low quality places to avoid. We worked with a GC that dealt with two modular home manufacturers in PA and they didn't deal in manufactured homes. Their quality was a step up. Our home is a Ritz Craft and I couldn't be happier. The after installation support from Ritz Craft exceeded out expectations. They came out and fixed every issue we found and many that they noticed which we didn't! Definitely upgrade where it makes sense. We did Anderson 400 series windows and had a local flooring company put in the hardwoods.
Modular homes are built stronger than stick built because they need to handle the shipping. The downside is that you have some design restrictions. IF you can handle the design restrictions, it should be a no-brainer to go modular. Our GC pointed out the restrictions to us as we worked together on the design of the house. They constantly told us that they could do anything, but the more the GC did instead of the factory, the more costly it would be.