I have the RDR-HX970 and have been very pleased with it, but there is one thing I am not clear about. At present I am using the Guideplus EPG, the main reason being that I can use Videoplus and PDC on Channels 1-5. Does EPG Link serve the same purpose as PDC and, if so, is it as reliable, or am I better sticking with Guideplus?

PDC is only available on analogue broadcasts.
Do you have an external receiver e.g Sky digibox,cable stb connected to the Sony RDR-HX970,if so then the Guideplus epg is want you'll need to use.
If however you are using only the built in Freeview,then the digital epg is ideal,as Series link will record all episodes of your selected programmes.

No external receivers, just the built in Freeview. I haven't used series link yet, but my real question is whether EPG link is as reliable as PDC.

PDC is not 100% reliable and neither is the EPG linked recording.

The main 5 channels are mostly reliable, all others less so.