For a little of the Torah thinking on nusach read this:
http://www.torah.org/advanced/weekly-halacha/5760/kiseitzei.html.
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Harav M. Feinstein was asked numerous times for his opinion on this matter. For reasons which are not completely clear, he did not always give the same response. Basically, though, he was unsure if the Biblical prohibition of Lo sisgodedu applied to the question of different nuschaos - but the Rabbinical edict against deviating from local custom was definitely relevant. Usually, he advised that the local custom be strictly upheld, unless the individual could keep his practiced undetected by others in the shul. The following is a digest of the guidelines that Harav Feinstein offered on this subject and its specific applications [see footnotes for the opinions of some other poskim]:
Any part of davening, e.g., Shemoneh Esrei, which is normally recited in an undertone may be recited in one's own nusach(16).
Preferably, Pesukei d'Zimrah and Birchos Kerias Shema should be recited according to the nusach of the minyan, since they are sometimes recited out loud. If, however, it is difficult for one to change from his own nusach, they may be recited in one's own nusach provided that they are said in an undertone(17).
Kedushah or any other part of davening which requires a minyan should be recited according to the nusach of the minyan, even if it is recited in an undertone(18).
Viduy and the Thirteen Midos should be recited with the congregation before Tachanun, even if one's own custom is not to do so(19). One may recite Viduy in an undertone in a shul that does not recite Viduy, but one may not strike his chest as he usually does. The Thirteen Midos cannot be recited, since a minyan is required to recite them(20).
A sheliach tzibbur must always daven according to the nusach of the tzibbur, even while reciting his own silent Shemoneh Esrei(21).
On Pesach night, one should recite Hallel with the tzibbur even if his custom is not to do so; he may not leave shul to avoid reciting Hallel. If he can avoid reciting the blessing without it being noticeable he should do so; otherwise he should recite the blessing as well(22).
One who follows the view of the poskim that yire'u eineinu is not recited at the nightly Ma'ariv, need not recite it with the congregation if the people around him will not notice his omission(23).
If the officers and members of a shul do not mind, there is no halachic objection to having men who put on tefillin on Chol ha-Moed and men who do not, daven in the same shul(24).