Hi, actually both sets are re-issues. This material was originally issued on separate LP's in the 1960's, chronologically starting with Symphony no. 8 on SXL 6044 and ending with Symphony no. 9 on SXL 6291. The SXL 6515-21 catalogue number puts this set around 1971 as you say, but the D-series boxes are later, from ca. 1976.
However, please keep in mind that earlier is not necessarily always better. Much will depend on the matrix and stamper numbers and of course the vinyl condition. Assuming both are UK pressed (there are inferior Dutch pressing of very late SXL's and many of the D-series) the SXL set should probably sound better, but not by much.
If it is sonic superiority you're after, you should really make an effort to collect the separate LP's, preferably in their earliest pressings with wideband labels.
Good luck and enjoy!
However, please keep in mind that earlier is not necessarily always better. Much will depend on the matrix and stamper numbers and of course the vinyl condition. Assuming both are UK pressed (there are inferior Dutch pressing of very late SXL's and many of the D-series) the SXL set should probably sound better, but not by much.
If it is sonic superiority you're after, you should really make an effort to collect the separate LP's, preferably in their earliest pressings with wideband labels.
Good luck and enjoy!