Sicilian Scheveningen versus Najdorf: What are some of the characteristics that differentiate the two openings? I would like to compare the Sicilian Scheveningen

[FEN ""]
1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 e6


with the Sicilian Najdorf

[FEN ""]
1.e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 a6


Please note that I am only concerned about positions where the two openings share a common pawn structure; meaning Black has pawns on e6 and d6 and not on the positions in the Najdorf where Black has played an early ...e5. In the lines where these two openings share a common central pawn structure, what are some of the characteristics that differentiate the two openings regarding piece placement and Black's minority attack on the queenside with a7-a6 and b7-b5? I am mostly interested in contrasting different middlegame plans and tactics based on piece placement such as the knight at b8 going to either d7 (typical of Najdorf) or c6 (typical of the Scheveningen) as an example.
I just adore the Najdorf. So I cannot help favoring it over the Scheveningen. Yes, these two variations differ. They differ because white uses a bit different attacking strategies. The great thing about the Najdorf is that there are excellent ways of counter-attacking. Black does choose between e7-e6 and e7-e5 in the different lines. For example, here is a line where black chooses e7-e6

[FEN ""]
1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6 6.Bc4 e6


I think that by looking at the main lines of each variation will answer your question in the best way. For Najdorf, I can say that the Nb8 usually goes to d7. Yet the exact placement of pieces and plans depends fully on the plan that white chooses. For example, white has the classical attack with Bg5, f4 and Qf3.

[FEN ""]
1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6 6.Bg5 e6 7.f4 h6 8.Bh4 Be7 9.Qf3 Qc7 10.O-O-O Nbd7


In general, the sharp variations lead to concrete positions, where the best options differ from position to position. If white chooses to be aggressive in either of the two variations (Scheveningen or Najdorf), you will get sharp and dynamic positions where a single error will be fatal. If white chooses more calm paths, I think the two variations are very similar in their piece placements and typical ideas (e.g. the exchange sacrifice Rc8xNc3). My advice is to get a solid overview of all the lines in the two variations and go through at least ten grandmaster games for every line.