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Boxed are supposedly stronger I've heard, but it's very minimal in my opinion. Plus they make good jacking points also. I run UMI's tubular Subframe Connectors and I am happy with these.
Boxed is stronger though tubular is not going to to be as heavy or take up as much space (clearance) . Not that here is anything wrong with either it just depends on your application and there should not be that big of a difference .