Which is a better way to check if an array has more than one element?

Use this

if (sizeof($arr) > 1) {
     ....
}

Or

if (count($arr) > 1) {
     ....
}

sizeof() is an alias for count(), they work the same.

Edit:
Answering the second part of the question:
The two lines of codes in the question are not alternative methods, they perform different functions. The first checks if the value at $arr['1'] is set, while the second returns the number of elements in the array.