The last post explains the relationship between the different kinds of pencils:
The rotring 300 is rotring's entry level pencil in the numbered series of pencils. The next up is the 500, then the best known of the series, the 600 followed by the rapid PRO and then the top of the line 800. At one time, when the pencils were made in Germany there was a 400 and a 700 in the series. However these are no longer produced and fetch a pretty penny when found. The rapid PRO, cost wise, fits in between the 600 and the 800. This sort of fills the gap left by the out of production 700. There does not appear that there is a replacement for the 400.
I own at least one pencil of each category, namely 300, 500, Rapid Pro, and 800. These are all 0.7mm. As I do little drafting, the 0.35 (or 0.3, they are the same) is useless to me, and I do find the 0.5 is still to thin for writing. However, none of these is my favorite pencil for underlining, note-taking, and writing . I like the Newton mechanical pencil, often referred to as a 600 (which is, however, false):
It also is a 0.7mm. And I prefer F or H for hardness, as HB or B smudges far too easily.
The prices demanded nowadays are outrageous. I bought three black and one silver pencil for the same price that people now want for one.
1. There are also Rotring "wannabes". I count among those the TWSBI Precision, the Retro 1951 Hex-O-Matic, also reviewed in the blog mentioned, and, of course the Redcircles which come in every size as well.