Q:

What's more efficient, array formulas or individual formulas?

Simple question: 
When adding, multiplying, or otherwise doing simple operations on two ranges is it better to use array formulas or individual formulas? 
=A1+B1
=A2+B2
=A3+B3
vs 
{A1:A3 + B1:B3} 
Array Formula Advantages: 

Can't edit part of an array (less error prone)
Maybe easier to understand? 

Single Formula dragged down the range

This is what people typically do

Can't find any guidance so maybe it really doesn't matter from a performance standpoint? Is it only personal preference? 

A:

The most significant difference is that with array formulas you can calculate things which are not possible to achieve using standard formulas.
For example, using SUM() with array formula, you can sum products!
{=SUM(B2:F2*B3:F3)}

This is out of reach of standard formulas.
For more, please watch short intro to array formulas.
Now, if you are asking which are more efficient: each type has its own application, we can stop comparing there. But if you want to use them for the same task, array formulas can be a bit faster, but a bit more difficult for editing. For example, you can expand range of array formula by mouse, but you cannot shrink it etc. As a side effect, you can't easily overwrite array formula ranges. Is it better? For someone, perhaps. But this was attempt on "comparison" of two things which are actually a bit different in their nature.

