There doesn't appear to be any difference in the results of using either cat or dd to create an image of a partition. Both the commands
cat /dev/block/nandg > /sdcard/nandg.img
and
dd if=/dev/block/nandg of=/sdcard/nandg.img
produce exactly the same results (a byte comparison shows the files are identical in size and content). dd has more options (block size, start byte, bytes to read, etc) so may be more useful.