The only problem with the diploma course is that it doesn't show evidence of any site work: you only have college-based practical assessments. So it's not quite as good from an employability point of view as the level 2 NVQ, but, as you say: it's a foot in the door, and you are still very young. If you have some site experience, you may find employers will look at that as a strong point in your favour.
Plumbing Studies (what a crap name!) seems to be a new name for the old Plumbing Craft diploma? The syllabus looks to be similar. Weirdly the City and Guilds website still seems to use the old name, unless I'm on an old website.
Academically, the plumbing craft diploma was much the same as the NVQ. Possibly slightly better, in fact. Bear in mind you can pass the multiple choice tests with minimal knowledge so do try to get the most out of the course and learn the whole whack of the textbook and as much additional information as you can get out of your teachers. And I hope you enjoy it!