The first thing to say is that whatever you think of these performances, be careful what you pay for a disc amounting to only 41 minutes of music. I find one of the works much more successful than the other, and I actually suspect that the composer may have as much to do with that as do the performers. The Magnificat (here in its more familiar D major version) simply does not strike me as being equal in inspiration to the best of the cantatas, such as no 51, and to whatever extent this may be a valid view the recording (certainly) and the performance (possibly) conspire to reinforce it.
You may experience a slight problem in setting the best volume-level for the Magnificat. The setting that suits the arias best may make the opening chorus uncomfortably loud, and at any level the solo voices seem a little backward. This is not as serious a matter at it would be in Handel or Mozart, since Bach's inspiration is basically instrumental with the voices integrated into the instrumental pattern, and in fact my ear became accustomed to it quickly. All the same, I did not sense quite the level of inspiration and fervour that I have found in most of Gardiner's Bach performances. Everything is basically `right', and these performers are eminent Bach stylists, but they have set the standard of comparison that I go by, and I have heard them in better form than here. The choral sound in particular lacks the ultimate degree of clarity that I would have liked and that I am used to from these singers.
Cantata 51 is another matter entirely. This is simply superlative. I adore Emma Kirkby's voice, she is at her very best here, the recording has brightness and immediacy, and the whole performance is on fire. The only date mentioned in the liner is 1983, so presumably that was the year of both recordings, which makes the discrepancy in quality a little puzzling. The liner essay makes the excellent point that Cantata 51 is a kind of solo motet, Italian in origin, and I am familiar with the style from the much more Italianised Handel. The German school had by Bach's time naturalised the style completely, and to complete the Lutheran feel of the work Bach inserts a long and most beautiful and elaborate chorale. What the liner has to say about the music is very good and enlightening, so more's the pity that it squanders so much of its space in telling us that the music does this then that then the other, all of which we can hear perfectly well for ourselves.
4 stars may be slightly generous, given that the amount of music provided is far from generous. However it's possible that no performance at all could turn the Magnificat into Bach's finest effort, and there is nothing seriously amiss with this performance anyway. As for Cantata 51 as done here, I would have paid any asking price for it.