Whoever has more lands in hand is more likely to win, but how many lands you draw off the swans will still matter.
Say I have 5 lands and you have 4.
I target the swans with Seismic Assault, discarding one land. If you let it resolve, I draw two cards and potentially gain even more land in hand advantage. If you try to respond by activating your own Seismic Assault, I can respond by discarding another land. Since I have land advantage, my Seismic Assault will always resolve first, which means I get to draw cards first. Whoever draws cards first, if he draws at least one land, he can activate Seismic Assault again to keep the chain going. There will be an opening if the player with land advantage is unlucky enough to draw enough nonlands to lose his advantage.