Rectangular. You need to be able to get around it easier. Should also be able to use as an extra cooking or serving area while people are on the other side. Consider a narrow one with wheels so you can move as needed. I recently had similar space for daughter's first apartment. Ikea!

Add this black marble table, decorated with inlaid gemstones and other natural stones. We manufacture these items and export around the world

You have a cute little apartment there! I can see space is at a premium, and I don't see a spot for a regular size table. However, if you move those two small round tables (or ottomans? Hard to tell) to another spot, you could fit a small rectangular bar table like the one shown below (Ikea). It would snug up to the wall and the. Come out into the room 4', acting as a divider. You could fit two bar stools at it and, bonus, you could watch TV while eating! The other option would be a coffee table like the one in pic three (West Elm) which opens out to become a kind of TV tray, or laptop table as shown in the pic. Yet another option is to buy a small "C" table that can be either a side table or can be pulled up in front of the sofa, again, to act as a TV tray. I do not see any other obvious spots for any kind of table in your space, so I hope one of these ideas can work for you!

I would go round and simple;)

Round, and small to fit the area.

I think it depends on how much you entertain honestly. The idea of a 6 seater is great but is it practical? You should consider how you will ge the most use out of your space. Maybe a couple club chairs and a small table would work

Maybe you'll be interested in a such dining set? The table crafted from dark wood and features a smooth finish. Dark colored dining table creates incredible atmosphere of sophisticated fashionable style.
http://www.furniturenyc.net/product/wooden-extendable-dining-table