So the LXX is not just a translation of the Hebrew OT; it is in itself a divinely given moment in the process of divine revelation; in a sense, rather like the discernment by the Church of the Canon of Scripture. It therefore deserves respect for and in itself, and is neither only nor even mainly a means to a different end (such as the reconstitution of a Hebrew 'original text').
But that concept of an 'original text' is, as I observed earlier, an idea characteristic of the Enlightenment but in itself questionable and now questioned. I think it can be sustained best in relation to an epistle of S Paul (there must presumably once have been one particular document which physically was taken by Phoebe from Corinth to Rome). But, even here, there is the overwhelming probability that all our existing textual forms go back to an early collection or edition of the Apostle's writings. Once you move beyond the Epistles, you run up against the relationship between Orality and Literacy in cultures predating the invention of printing, and particularly in the ancient world. Work has been done on this subject, both by secular Classicists (such as Rosalind Thomas of Balliol) and by NT specialists (such as Loveday Alexander at Sheffield). To put just one part of this briefly: in a fundamentally oral society, the written word often served as back-up for business which was mainly done orally. If you taught somebody cookery, this was basically done on the job, by word of mouth, in the kitchen. Books about cookery were supports, but they presupposed the oral and, in reaction to the oral, were texts that tended to fluidity. (You may yourself have a cookery book in your kitchen which, over the decades, you have modified, corrected, augmented as the result of your own practice of the culinary art.) Even in the letters of S Paul one finds hints that the person who (physically) carried the letter will fill it out, will explain it to the recipients.
So the 'Enlightenment' idea that, if only you had enough evidence and sufficient skill to deploy it, you could in principle reconstruct an 'original text', is dubious (it also puts disproportionate power into the hands of those who proclaim themselves to be Experts, and whose 'scientific' conclusions will probably be overturned by the generation which succeeds them). Even more dubious is the common Protestant superstition (a superstition because it erroneously makes into an idol, reifies, what should be one functioning element in ecclesial life) or fetich (a fetich because it is a paraphilia rather like being erotically fixated on your husband's ears rather than on his totality) that there is a static 'Bible' which stands as a test of doctrine over and above the life of the Church, and to which that life is subject and, even forensically, needs to be made answerable. 'Bible' is simply a vitally important element within a whole, within a traditio or paradosis. And this should, in my opinion, lead us to a privileging of those biblical editions which have fed and do feed the Church, have been cited by Fathers and Councils, and have been sanctified and authorised by sustained liturgical use. So: three cheers for the LXX.
And ... my final point ... three cheers also for the Vulgate*. And I would include in my cheers the passage about the Adulterous Woman, in John 8, even if it is not an 'original' part of the Gospel, and 1 John 5:7b, even if that is not part of the 'original' text of its Epistle, and the last part of Mark 16; such passages, whatever their history, are still canonical Scripture. Incidentally, by Vulgate (Vg) I do not mean the NeoVulgate of S John Paul II, which I regard as subordinate to the 'real' Vg because of the 'Enlightenment' methodology of its production. There is most certainly nothing bad about it; it has the Church's formal approval. It just does not have the status, the auctoritas, of the LXX or the proper Vulgate (I suppose, a thousand or two years of intensive use might enhance the status of the NeoVulgate!). And, happily, the LXX and the Vg present us with texts which have considerable similarities. It's not nearly so often a matter of LXX versus Vg as it is of LXX+Vg versus The Rest. (The day, incidentally, when Orthodoxy abandons the Textus Receptus will be the day when, I hope, my Orthodox friends will become Old Believers!)
So don't throw away your English translations of the Vulgate, whether they be Dr Challoner's revision of the Douai-Rheims Bible, or Mgr Knox's translation, sadly underrated as it nowadays is. There is certainly no harm in the RSV (make sure that it is either a 'Catholic Edition' or else contains the 'Deuterocanonical Books', and do not ever use the feminist "New Revised Standard Version") ... it is probably the best of the modern Anglophone Bibles and it is certainly better to read the RSV than to read nothing ... but ... well, I've given you my own preferences!
* I do not include in the same three cheers the MT as used in the medieval and modern synagogue, because its text-type has been formed, for nearly two millennia, independently from and, to a degree, probably in reaction against, the Church. It has in its own right, of course, immense value and interest as a witness to the history of the post-Jamnian rabbinic Judaism of our present world, the product of that radical reconstruction which Diaspora Judaism needed after the destruction of the Jerusalem Temple had rendered so much of the Jewish Bible obsolete.