Continue the Attack, Or Meet the Positional Threat? In the league last night the following position came up:

[Date "2017.10.26"]
[White "Ian Bush"]
[Black "Not Ian Bush"]
[Result "1/2-1/2"]
[StartPly "28"]
[FEN ""]

1.e4 e6 2.d4 d5 3.Nc3 Bb4 4.e5 c5 5.a3 Bxc3+ 6.bxc3 Qc7 7.Nf3 Nc6 8.Bd3 c4
9.Be2 Bd7 10.O-O O-O-O 11.a4 f6 12.Bf4 f5 13.Bc1 h6 14.Ba3 Be8 15.Qb1 
    ( 15.Nd2 Qa5 16.Nb1 Nge7 17.Qc1 )
15...g5 16.Qb2 
    ( 16.Nd2 )
16...Bh5 17.Rfb1 Bxf3 18.Bxf3 g4 19.Be2 h5 20.a5 Nh6 21.Bd6 Qd7 22.a6 
    ( 22.Bc5 a6 23.Qc1 Nf7 24.Rb6 )
22...b6 23.Qb5 Nf7 24.Bc5 Qc7 
    ( 24...Ncxe5 $1 )
25.Bxb6 $1 axb6 26.a7 $2 
    ( 26.Qxb6 )
26...Nxa7 $2 27.Qa6+ Qb7 28.Qxa7 Qxa7 29.Rxa7 Rd7 30.Ra6 Rb7 31.Rbxb6 Rxb6
32.Rxb6 Kd7 33.Rb7+ Ke8 34.f3 Rh6 35.Rb8+ Ke7 36.Rb7+ Kf8 37.Rb8+ Kg7 38.
Rb7 Rg6 39.Kf2 Kf8 40.Rb8+ 1/2-1/2


I played 15. Qb1 and after various adventures and a fun game it ended a draw. And Qb1 is Stockfish's choice in this position, followed by the related a5, Qd2 and Qc1, in human terms it agrees with my plan of blasting through the Queen side. 

However on reflection in the cold light of day I'm wondering if 15. Nd2 (as indicated) may be better to first meet the positional threat he has of swapping off his bad bishop for my good knight via Bh5 (also relieving a little cramp in has camp). Yes I lose a tempo or two for my attack, but I'll get some of it back when he has to move his queen back from a5, and I gain the opportunity to sac a piece for 2 pawns on c4 so bringing a second minor piece into the attack - and so giving my white squared bishop at least some purpose in life, even if it is only self-immolation. 

So what do people think is better Qb1 or Nd2? Is this the kind of long term thing that computers aren't that great at? Or should I just trust the digital monster?
I think you are asking the correct question at the wrong move.  12.Bf4 is probably questionable and after 12...f5 you need to think about countering Black's kingside expansion.  A move like 13.Ng5 looks strong, a possible line being 13...Rf8 14.h4 h6 15.Nh3.  Then you can reroute the Bishop, put the pawn on h5 and Nf4 with a bind.

It is probably too late to thwart Black's light squared bishop with 15.Nd2 (which just looks ugly to me) as you are now having to deal with a kingside pawn storm but 15.Ne1 looks much better.  The idea is to play g3 and reroute the knight via g2 at some point.