Comparing 53P63S66 to 53P21S66 is an interesting comparison. As is comparing 53P63S66 to 42P63S66.
53P63S66
  White is Player 2
score: 0 pip: 158  Unlimited Game Jacoby Beaver  pip: 159 score: 0
Blue is Player 1  
XGID=aBDaBdEaceB:0:0:1:66:0:0:3:0:10 
Blue to play 66 
1.  XG Roller+  13/7*(2) 7/1*(2)  eq: +0.865 
 Player: Opponent:  67.60% (G:39.83% B:1.23%) 32.40% (G:7.09% B:0.33%)  

2.  XG Roller+  24/18(2) 13/7*(2)  eq: +0.837 (0.028) 
 Player: Opponent:  68.84% (G:21.54% B:0.89%) 31.16% (G:5.69% B:0.19%)  

3.  2ply  13/7*(2) 8/2(2)  eq: +0.658 (0.206) 
 Player: Opponent:  63.29% (G:28.97% B:1.49%) 36.71% (G:8.32% B:0.35%)  

4.  2ply  24/18 13/7*(3)  eq: +0.653 (0.212) 
 Player: Opponent:  64.00% (G:25.79% B:1.67%) 36.00% (G:8.11% B:0.33%)  

5.  2ply  24/18(2) 13/7* 7/1*  eq: +0.630 (0.234) 
 Player: Opponent:  63.12% (G:24.69% B:1.04%) 36.88% (G:7.67% B:0.27%)  

eXtreme Gammon Version: 2.10
53P31Z66
  White is Player 2
score: 0 pip: 163  Unlimited Game Jacoby Beaver  pip: 159 score: 0
Blue is Player 1  
XGID=aaBDBdEaceB:0:0:1:66:0:0:3:0:10 
Blue to play 66 
1.  3ply  24/18(2) 13/7(2)  eq: +0.739 
 Player: Opponent:  66.76% (G:21.06% B:0.77%) 33.24% (G:6.64% B:0.19%)  

2.  3ply  13/7(2) 8/2*(2)  eq: +0.651 (0.088) 
 Player: Opponent:  62.96% (G:29.40% B:1.56%) 37.04% (G:8.15% B:0.39%)  

3.  3ply  24/18(2) 8/2*(2)  eq: +0.616 (0.122) 
 Player: Opponent:  63.26% (G:24.57% B:1.20%) 36.74% (G:7.56% B:0.31%)  

4.  3ply  13/1* 8/2*(2)  eq: +0.600 (0.138) 
 Player: Opponent:  61.33% (G:32.22% B:1.19%) 38.67% (G:9.42% B:0.51%)  

5.  2ply  13/1*(2)  eq: +0.548 (0.191) 
 Player: Opponent:  59.96% (G:28.78% B:1.11%) 40.04% (G:9.64% B:0.46%)  

eXtreme Gammon Version: 2.10
1 checker on the bar instead of 2 is a huge swing against the attacking play. Your opponent has a lot more chances just to anchor up and kill your blitz stone dead.
53P63S66
  White is Player 2
score: 0 pip: 158  Unlimited Game Jacoby Beaver  pip: 159 score: 0
Blue is Player 1  
XGID=aBDaBdEaceB:0:0:1:66:0:0:3:0:10 
Blue to play 66 
1.  XG Roller+  13/7*(2) 7/1*(2)  eq: +0.865 
 Player: Opponent:  67.60% (G:39.83% B:1.23%) 32.40% (G:7.09% B:0.33%)  

2.  XG Roller+  24/18(2) 13/7*(2)  eq: +0.837 (0.028) 
 Player: Opponent:  68.84% (G:21.54% B:0.89%) 31.16% (G:5.69% B:0.19%)  

3.  2ply  13/7*(2) 8/2(2)  eq: +0.658 (0.206) 
 Player: Opponent:  63.29% (G:28.97% B:1.49%) 36.71% (G:8.32% B:0.35%)  

4.  2ply  24/18 13/7*(3)  eq: +0.653 (0.212) 
 Player: Opponent:  64.00% (G:25.79% B:1.67%) 36.00% (G:8.11% B:0.33%)  

5.  2ply  24/18(2) 13/7* 7/1*  eq: +0.630 (0.234) 
 Player: Opponent:  63.12% (G:24.69% B:1.04%) 36.88% (G:7.67% B:0.27%)  

eXtreme Gammon Version: 2.10
The made 3point is deep enough to favour a blitz approach over a more pure positional approach.
42P63s66
  White is Player 2
score: 0 pip: 158  Unlimited Game Jacoby Beaver  pip: 161 score: 0
Blue is Player 1  
XGID=aBDaBdEaceB:0:0:1:66:0:0:3:0:10 
Blue to play 66 
1.  XG Roller+  24/18(2) 13/7*(2)  eq: +0.928 
 Player: Opponent:  71.36% (G:20.72% B:0.90%) 28.64% (G:5.01% B:0.15%)  

2.  XG Roller+  13/7*(2) 7/1*(2)  eq: +0.897 (0.031) 
 Player: Opponent:  68.86% (G:40.93% B:1.35%) 31.14% (G:6.56% B:0.30%)  

3.  3ply  24/18 13/7*(3)  eq: +0.796 (0.132) 
 Player: Opponent:  66.62% (G:27.45% B:1.76%) 33.38% (G:7.11% B:0.27%)  

4.  2ply  13/7*(2) 8/2(2)  eq: +0.746 (0.181) 
 Player: Opponent:  65.11% (G:30.45% B:1.74%) 34.89% (G:7.84% B:0.34%)  

5.  2ply  13/7*(3) 7/1*  eq: +0.693 (0.235) 
 Player: Opponent:  64.06% (G:31.91% B:1.51%) 35.94% (G:8.60% B:0.46%)  

eXtreme Gammon Version: 2.10
By contrast the made 4point favours the positional play.
I guess the simple lesson is that when you've already made deeper points your blitzing potential goes up and your priming potential goes down so you should favour blitzing. When you've made higher points you can play a more balanced game.
If we look at 64P63S66 and 31P63S66 we see this analysis confirmed.
With the made 2point blitzing is even more right than with the made 3point. With the 5point not blitzing is more right than with the made 4point.