A PGDip is a lower qualification than a Masters. A taught Masters carries 180 credits, consisting (generally) of 120 credits from taught unit coursework/exams/tests etc and 60 credits from a dissertation. A PGDip is the taught Masters unit results only, minus the dissertation.
Some universities allow you to enrol on a PGDip course. This tends to be the case for people with an undergrad result too low for full Masters entry. Some unis then allow progression to the full Masters if PGDip coursework is good enough. At other universities, a PGDip may indicate that taught coursework has been passed but the dissertation has been failed. The PGDip can therefore carry a potential - if sometimes unfair - connotation of failure at Masters level.
As to which gives greater employment opportunities, that very much depends on the field you're aiming for. Very few jobs will specify a Masters as an entry requirement, so you need to research your target career to work out whether the investiment in either a Masters or PGDip is worthwhile. Also, you may find that some industries aren't familiar with the PGDip as a stand-alone qualification so that may not help you at all.