Go with the 50x50RHS IMO. It will be so much stiffer you would not believe, presuming the joints are welded of course. Just make sure the RHS you buy is as straight as possible. I'd like to see a design for a 5kg gantry spanning 1250mm, think you might have to defy the physical laws of the universe to achieve it but I'm always eager to be educated :)

Don't be too afraid of weight in either your table or gantry, the mass will absorb a lot of the harmonics generated by the router and you wil get much smoother and accurate cuts.

Just my opinion, probably get shot down now :)

Jeff.