This is the USA and almost worldwide standard for W&M 14g, 0.080", or 2mm (all the same) so that's what you'll get if you buy any 14g nipples. There are other threads, but they're mostly obsolete so there's no need to consider them unless dealing with a vintage bicycle, not from the US or UK.

15g spokes use a special thread with the same pitch, but with a smaller diameter. There's never a danger of using 15g nipples on a 14g spoke since it simply won't fit. OTOH it's possible to be fooled and use 14g nipples on a 15g spoke. The nipple fits and seems to thread on normally, then surprise you by slipping back as you approach tension.

As for thread quality, all the nipples I've worked with over the last 20-30 years have formed threads which are very smooth, and I doubt you'll find variation in thread quality changing the nipples unless there's corrosion.

The other consideration is length. Nipples come in various lengths and more important various thread lengths. If the thread length is too great, the nipple can jam when threaded on too far. Some nipple/spoke thread length combinations will jam well before the spoke reaches the head. If you're finding rough threads it might be that you've run out of thread.