I think you will find the function randperm useful. You can use it to generate a random order for your 150 rows and then select the first 30 for your smaller set and the last 120 for the larger set (order not important).
For example
my_indices = randperm(150);
small_set = data(my_indices(1:30),:);
large_set = data(my_indices(31:end),:);
