Does variable size definition affect SIMULINK performance? Vector vs Scalar

I have a SIMULINK simulation and as I understand if a define the size of a I/O as let's say it, 3, it means it is a vector, but SIMULINK recognizes that it only has one dimension.

Now, if I declare the I/O as [3 1], it is also a vector but this time it has two dimensions. More over signals defined with 3 and signal with [3 1] cannot be used together in some cases.

So, my question is: which way should I use? Does SIMULINK performance is affected by this convention?

[Walter Roberson]

There are portions of Simulink that treat an explicit dimension of 1 as meaning "variable length". So [3 1] might be interpreted as a 2D array that is 3 rows and an indefinite number of columns.