The short answer is neither.
Patents must be filed within one year of the first public use or offer for sale. Too late.
Trademarks won't protect the furniture designs themselves, only the name/phrase you use to designate yourselves as the supplier. Here are some examples of trademarks so that you might get the idea: IKEA, Eames, Barcelona (chair), Aeron (chair), etc.
While trademarks won't protect the designs themselves, you can see from the examples above that trademarks can be quite valuable and important.
Other things you might consider are trade dress/unfair competition and copyrights.
Trade dress/unfair competition. This is a fairly close relative of trademarks. They pertain to trying to fool the public by making one product look and/or feel confusingly similar to another product. Suppose I made Jimbo Cola and packaged it in red cans with a white/silver swoosh/wave. Suppose also that Coca-cola didn't trademark the color or swoosh design (they may have for all I know). It's arguable that my design is too similar to that of Coca-Cola's so I might be unfairly benefiting from consumer confusion. That's trade dress.
Now, notice that I focused on the can (i.e., packaging), not the product itself. It's perfectly okay that Jimbo Cola is brown and carbonated. It's also perfectly okay if Jimbo Cola tastes exactly like Coca Cola, assuming I didn't have unauthorized access to their trade secrets. I don't know under what circumstances, if any, trade dress can apply to the product itself and not just its marketing/packaging.
Trade dress is a subset of unfair competition, I think. Unfair competition can include other types of behavior that really isn't pertinent here.
Copyrights. This could be a bit of a stretch, but it's one of my favorite clever arguments. To the extent you can claim that your great grandfather's designs were artistic scultures, you can claim copyright in them. The good news there is that you may already be protected -- however, copyright law was different decades ago so I'm not clear on that. The bad news is that copyright protects against copying, not independent creation or even works "inspired" by copyrighted works. Well, up to a point -- at some point, a subsequent "inspired" work is considered "derivative" and is protected.
I hope that helps.
Regards.