The first important point here is that you at least see the possibility of playing 5/off, 5/2. Many players would automatically make the 'forced' move 6/1, 6/3. How to evaluate such a position? The first thing is to look at the number of cross-overs (a cross-over moves a man from one quadrant of the board to the next or off the board). Here Black needs 11 cross- overs and White also needs 11 (four to bring his man on the bar to his home board and then seven to bear off). This means the position is close.
The other key factor is the blot on White's 2-point. If Black plays (b) and White enters with 22, 23, 24 or 26 hitting Black's blot then Black in turn will have the chance to hit White's blot, gaining significantly in the race. If White enters with 25 he should still hit the blot on Black's 2-point by playing Bar/23*/18 rather than play Bar/20/18.
The other benefits to (b) are that it takes a man off and maintains the 6-point for another roll. Compare the positions if White rolls 64 after Black has made his play. In case (a) White is a big favourite, in (b) he is still on the bar. The downside of (b) is that White may hit and then Black may miss, in which case White becomes a very big favourite.
Over the board it is difficult to balance these factors but, as I have said before, when in doubt be aggressive. I chose play (b), my opponent stayed on the bar with a 63, but backgammon being backgammon I lost when he rolled 66 with his final shake of the dice. Jellyfish analysis shows that Black will win the game 71 per cent of the time after play (b) but only 67 per cent after (a).