The advantages of getting into top classes allow the child to study at a faster pace and possibly stretch one’s potential when you study with the higher ability group and in a so call more competitive environment.
However, it is not a guarantee that one will score above 250 in PSLE. For some unknown reasons, there are students in top classes who did not do well and scored 21x - 23x. An extreme case I heard of was below 200 and the need to choose between express and normal.
If I am seeking a transfer for my younger one, I will look for 1 that has an affiliation to secondary (for security reason), otherwise, I do not see much advantage.
There's nothing quite like being able to discuss issues and network with other parents with children in the same school as yours. Individual Schools' Official Parent Support Groups are welcome to come in and help out the incoming groups of parents.