Firstly - obviously pre-pay is not better for everyone (i.e. those unlimited plans look awesome, if I needed them or had kids with phones etc.)

I see myself as a typical user, who is wanting more of a focus on data. I use less than 100 minutes per month.

Sim only plan vs Prepay:
$29 Plan       :       $19 Prepay
500mb         :       500mb
150m           :      100m
unlimited txt  :      unlimited txt
4G               :      4G

For the average consumer pre-pay freebees make prepay the much better option.  Prepay also get given 250mb if you top up $20, 500mb if you top up $30 and 1gb data if you top up $40.  So in reality, if you went pre-pay you would get:

$29 Plan       :       $20 Prepay      
500mb         :       750mb
150m           :      100m
unlimited txt  :      unlimited txt
4G               :      4G      
                   :       $1 credit left over each month to spend as you choose

OR

$29 Plan       :       $30 Prepay      
500mb         :       1000mb
150m           :      100m
unlimited txt  :      unlimited txt
4G               :      4G
                    :      $11 credit left over each month to spend as you choose   
                            (equivalent to another 22 minutes of calling)

$29 Plan       :       $40 Prepay      
500mb         :       1500mb
150m           :      100m
unlimited txt  :      unlimited txt
4G               :      4G
                   :       $21 credit left over each month to spend as you choose   
                           (equivalent to another 42 minutes of calling)

I would like Vodafone to give me the same freebees every time I pay them $30 too!