Hi,

I want to buy router. So which one is enough for my 120 square-meter home between RB951Ui-2HnD and RB951Ui-2nD routers.

RB951Ui-2nD : 1.5 dbi antenna gain and 22dbm max Tx power
RB951Ui-2HnD : 2.5 dbi antenna gain and 30 dbm max. Tx power.

Will I get any wireless issues if I bought RB951Ui-2nD because of it has 1.5 dbi antenna.

You should regulate the tx power according to your legal limitations anyway so it should look both the same. Rather consider rbd52g, which is more modern, more powerful and with two independent radios.
Better to use two low tx devices on sides than one strong in the middle.