Rabbi Moshe Feinstein, Rosh Yeshiva of Tiferes Yerushalayim (1895-1986)

Regarding your question if there is a mitzvah nowadays to live in Eretz Yisroel, as the Ramban says, or if there is no mitzvah nowadays, as Rabbeinu Chaim says, quoted in Tosafos on Kesubos 110b: Most poskim hold it is a mitzvah. But it is obvious that this is not an obligatory mitzvah in our time, for if so, it would result in a prohibition to live in Chutz Laaretz because doing so would be violating a positive commandment, like wearing a four-cornered garment without tzitzis, which is forbidden because it transgresses the positive commandment of tzitzis. But we only find a prohibition regarding one who lives in Eretz Yisroel, who is forbidden to leave with the intention of dwelling permanently in Chutz Laaretz - see Rambam Hilchos Melachim 5:9. And this prohibition is certainly not a negative commandment. Had it applied to the Jews of Chutz Laaretz, the Rambam would have written, 'It is forbidden to live in Chutz Laaretz unless there is a strong famine in Eretz Yisroel.' So we see that it is a Rabbinic prohibition, only for those who dwell in Eretz Yisroel. But as far as the Biblical positive commandment, it is not obligatory, only that if one lives there he fulfills the mitzvah. And in my Chiddushim I have written at great length about the words of Rabbeinu Chaim in Tosafos in Kesubos. Now, since it is not an obligatory mitzvah, you must definitely take into consideration the concern of Rabbeinu Chaim in Tosafos as to whether you will be able to keep the special mitzvos of Eretz Yisroel." (Igros Moshe, Even Hoezer 1:102)