21 is good only for high floors (see beyond 226A), wall of main bedroom will get bake by west sun for half the year, road noise.
31 better view but more road noise (main road entrance), service yard west sun (good), may get some noise from PP from time to time.
29 blocked view, less road noise, service yard west sun (good) but wall of main bedroom will get bake by west sun for half the year.
HDB is very smart nowadays (well design estates) because there usually isn't a single stack that is perfect. Just decide on what is more important to you and make the choice.