Hi Charlee, I Think I would go with a rectangular one simply for continuity.

If you look at round dining tables in Houzz photos, you'll see lots of rooms with other than round (actually more don't have round). It will help you to know which look you like better and my suggestion for a more modern look, as well as for the length is rectangle.

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There's also a much better selection of rectangular rugs than round.

You could also look for a square rug. There's quite a nice selection of 8' squares if that would be large enough for your table.