I am just coming to the end of the northern course after doing the southern course, and while I find both Aran and Catrin admirably clear speakers, I am interested to note certain differences in their pronunciation (which is, of course, only to be expected). In particular, I note that Catrin tends to pronounce the h in ‘nhw’ whereas Aran doesn’t, so with Aran ‘maen nhw’ sounds like a straight ‘ma noo’ whereas Catrin’s is more like ‘man hoo’. For some reason, this latter sounds sort of pleasantly posh to me. Now this is odd, because at this early stage of my attunement to the language, I would have thought I could have no possible basis for feeling that any pronunciation is posher than any other. I can only think it relates back to my English teacher at grammar school, a native Welshwoman who spoke the most beautifully correct English I have ever heard, and in particular aspirated words beginning with ‘wh’ to such a degree that her ‘hwhat’ and ‘hwhether’ could blow out a candle. This leads me to wonder if there are indeed strata of perceived poshness in Welsh diction similar to, say, the difference between Cockney and Oxford English or the old-fashioned and to my ear slightly wince-making ‘received pronunciation’ prevalent in the BBC of the nineteen fifties, but now thankfully replaced with a more democratic and varied delivery. I don’t know on what basis, but I have this possibly idealised view of Welsh as a language certainly with strong regional differences, but having little of the elocutionary snobbery that led George Bernard Shaw, in his preface to ‘Pygmalion’, to say ‘It is impossible for an Englishman to open his mouth without making some other Englishman hate or despise him’. No doubt you will tell me if I am wrong!
I would tend to say more formal/traditional than posh. When we were recording, Aran was always reminding me not to use my Cymraeg Capel (my chapel Welsh) and that I should be more relaxed and informal in my delivery. After all, the aim of the course is to provide a spoken form of informal Welsh.
I would sometimes be concentrating so much on getting the recording absolutely correct, that it would affect how relaxed I was, so would often sit bolt upright and slip back in to speaking more formally as if on a podium addressing an audience.
From day to day, I naturally use ‘nw’, unless I’m cross, or want to make a string point of something… 
Both ‘nhw’ and ‘nw’ of course would be easily understood. If you were to adopt the habit of using ‘nhw’ then people would assume that you either wanted to speak a more formal form of Welsh, or possibly that it was dialectical.
I used to try, as subtly as possible, to get Catrin to relax before recording.
It never really worked, and now we’re in a Wine Club, so that one rather got away from me…