So whats the relationship between transcriptional and post-transcriptional gene silencing?
The clear differences are these:
1. In TGS, a gene isn’t transcribed and all, while in PTGS, the gene is transcribed, but the transcripts are either truncated or degraded.

2. In TGS, the promoter is methylated (although it should be noted that some promoters are still transcribed even when methylated). In PTGS, the gene’s coding region tends to be methylated.

3. TGS is generally heritable, PTGS is generally not heritable.

4. And finally, there are Arabidopsis mutations that prevent PTGS without affecting TGS, uncoupling the two genetically.