In general the commemoratives command a greater premium in
un-mounted mint condition than hinged mint. Also, some of the sets are
especially tricky to find in fine used pairs. Some pairs have a habit
of splitting when being soaked off paper due to their long length.
The Voortrekker Memorial Fund has a nice variety on the 1d+½d, known as
the “Comet” flaw; it is a large red smudge across the stamp. The 1935
Silver Jubilee varieties can be expensive and hard to find. Be aware
there are 10 different settings of the ½d and 21 different settings of
the 1d 1936 JIPEX miniature sheets to find. The 1937 Coronation and
1949 U.P.U. issues have a multitude of minor and not so minor varieties
to hunt for. These are popular as they are not too expensive.