Going to visit both but think it would be better to day trip from one. Which is better to make out main place to stay?
Thanks
Brussels is where I would stay.
It depends on where you plan to day trip.
FYI--all public transportation in Luxembourg, including the trains, is free.
It would depend on your greater plans.
Do you want to visit Luxembourg castles, Trier and Bitburg just across the river, make a pilgrimage to the town of Schengen, want to visit Patton's grave, visit the casemates and Battle of the Bulge and the city, perhaps the Mosel (Moselle) river?
Or the Atomium, the EU quarter, Gent, Antwerpen, Bruges, perhaps a visit to Yres (Ieper)?
You can't do daytrips from Luxembourg to Brussels/Ghent/Bruges (it's too far). From Bruges to Luxembourg is like 4 hours by car and by train it's even longer. Even Brussels-Luxembourg is already like 3 hours by car. If you want to visit both, you should find a place to stay in both countries. It will be way more efficient.