Hrqls:

3&6: 27.8% chance of going out
4&5: 27.8% chance of going out

They are the same.

Not always the case. For instance let's say you have 7 pips left:

2&5: 52.8%
3&4: 47.2%
1&6: 41.7%

So 2&5 is much better.