Cost of assoc array?

[Chris via Digitalmars-d-learn]

I have code that uses the following:

string[][size_t] myArray;

1. myArray = [0:["t", "o", "m"], 1:["s", "m", "i", "th"]];

However, I've found out that I never need an assoc array and a 
"linear" array would be just fine, as in

2. myArray = [["t", "o", "m"], ["s", "m", "i", "th"]];

Is there any huge difference as regards performance and memory 
footprint between the two? Or is 2. basically 1. under the hood?