I have a lot of experience with the GIVI stuff, and no direct experience with the SW-Motech items...although I researched both before going with GIVI.
I think SW-Motech can't beat GIVI for strength...but how strong does the set-up need to be top accomplish what you want? Overall solidness may be a false issue, as the SW-Motech will probably be plenty solid.
Both allow use of side case racks if/when you want/need to go there
Price of the GIVI E370 monolock case with the SW-Motech top rack is very similar to the price of the GIVI box and GIVI FZ256 top rack, so that's not much of a decision-maker.
Some people say they like the SW-Motech racks because they can be attached/removed quickly. I can get all my GIVI racks on or off in less than 15 minutes. When they are on, they stay on for a good length of time. I haven't found a need to switch them on and off multiple times within a short span...so I've learned "easy-on/easy-off'' was a false issue in my case.
If I were you, I think I'd go with the SW-Motech rack(s) because you already prefer that look and you will probably be spending a lot of time with it after your purchase.
Those are all the issues that occur to me...I'd be interested in what your final decision is.