Why was 5...Nf6 traditionally preferred to 5...e6 in the Two Knights Caro-Kann? Wells (2007) p165 asserts that 5...Nf6 was traditionally preferred over 5...e6 because 

[FEN "..."]
1. e4 c6 2. Nf3 d5 3. Nc3 Bg4 4. h3 Bxf3 5. Qxf3 Nf6 6. d4 dxe4 7. Nxe4 Qxd4 8. Bd3 Nbd7! 


"is an unsound pawn sacrifice" whereas 

[FEN "..."]
1. e4 c6 2. Nf3 d5 3. Nc3 Bg4 4. h3 Bxf3 5. Qxf3 e6 6. d4 dxe4 7. Nxe4 Qxd4 8. Bd3


"is much more dangerous".

He goes on to state that in the latter line 8...Nd7! may overturn the old assessment, but the implication is that 6 d4 is still more dangerous against 5...e6 than against 5...Nf6.

Regardless of the current status of this line, what was the traditional explanation for why 8 Bd3 is more dangerous in the 5...e6 line than the 5...Nf6 line? Wells does not explain this. 

Wells, Peter (2007). Grandmaster Secrets: The Caro-Kann. Gambit Books.
After 5....Nf6 6.d4 dxe4 7.Nxe4 Qxd4 (1), 8.Bd3 seems the only move for white to prove he has enough compensation for the pawn. Now black has two good options: 8....Nbd7 or 8....Nxe4 9.Bxe4 Nd7.

After 5....e6 6.d4 dxe4 7.Nxe4 Qxd4 (2), white has more options. He can play 8.Bd3, but also 8.c3! is a popular option.

In terms of objective evaluation, both variants probably don't differ much, but black should have less problems in (1) compared to (2).

Should black play 5....Nf6 or 5....e6? Difficult to say.
According to the Game Database of ChessTempo, the latter is more popular, but, after white's most popular answer 6.d3, both moves often transpose. On the other hand, after 5....e6, black keeps the option to develop with g6, Bg7, Ne7.



      [FEN ""]
      [StartPly "9"]

      1.e4 c6 2.Nf3 d5 3.Nc3 Bg4 4.h3 Bxf3 5.Qxf3 Nf6 (5...e6 6.d4 (6.d3) dxe4 7.Nxe4 Qxd4 8.c3 (8.Bd3)) 6.d4 (6.d3) dxe4 7.Nxe4 Qxd4 8.Bd3 Nxe4 (8...Nbd7) 9.Bxe4 Nd7