Well, I've never opened either Realforce or Filco keyboards, but I still personally think Realforces are better built than Filcos.
They are definitely heavier, and whenever I type on them, I just get this super sturdy, solid feeling. Everything from the switches and the body itself seem more tightly put together.
Also, build quality includes things like switch quality, which are much better in Realforces than the Cherrry switches in Filcos. They are much more smooth, with a lot less wobble. I think they feel more solid, as well.
This is a video, I think, that highlight the differences pretty well:
Before he types on each keyboard, he swipes his fingers across all the keys, to show the differences in key wobble. I don't know if it's readily apparent in the video, but you can kind of tell the Filco's switches are a little more loose, and have a little more wobble.
And when he types, notice how they sound differently. The Filco sounds a little more hollow, probably because there is less in the body, vs. the Topre, which has a deeper sound, probably because it's heavier. Also, the Topre switches seem much more smooth, and more solid.