It really depends on how you use the space: if you're eating alone or with a partner most days, an extendable one is pure gold because you keep it small and take up little room, but the trick is you need somewhere to store the leaf that comes out (and a lot of small apartments don't have that, so it ends up taking up a corner anyway). Round tables flow better visually and people move with less friction around them, but here's the catch that nobody mentions: they take up almost the same diameter as the length of an equivalent rectangular one, so the real space savings is more psychological than mathematical. My recommendation is that you measure carefully where it's going to go, leave at least 60-70 cm of clearance on the sides so you can move around without stress, and if you don't spend much time at home or get visitors rarely, honestly a tall side table for eating on the go saves you more than anything else.