I think the long rectangular one is going to look less dated, and has the advantage of clean lines that help any space look a little more modern and structured and square-ish. (But if you love the other, go for it, just maybe consider it not a 20-year investmnet.)

I like them both, but I think the long rectangle is a bit more modern chic. However, I do think that it will be the one that will look dated. Plus you have the issues of dusting a fabric shade, and of putting two holes in the ceiling to hang that light. Later on if you want to change it, that is something to deal with.

All that said, if it were me, I would go with the rectangle and worry about the other issues later. I love the shaded pendants, and I think the rectangle makes a lot of sense over a rectangular dining table.

Of course, as a compromise, you could do a round pendant with a plain shade, instead of the leaf one.