...if you need a rain-sensor or a matre valve, now you need two...
Not completely true...
If the controller uses the rain sensors that simply connect in series with the "common", the you only have to connect the "common" of the two controllers together to use one rain sensor. (An electrical skematic would be required to determine if two controllers could share a single rain sensor if the controllers have seperate rain sensor inputs).
You also would not require two master valves. You can build a relay circuit that would allow you to run the master valve when either controller is on...but that requires some knowledge, and the cost of parts might offset the price difference in controllers.
You might also want two controllers if you want to allow two zones to be on at the same time. For example, I have both lawn irrigation zones and drip irrigation zones. I wanted to be able to run both lawn and drip zones at the same time. MOST controllers are not going to let you run two zones at once. So I opted for two cheaper controllers (one for lawn, the other for drip).
But unless you've got an unusual situation, I would agree with the general notion that one controller will be better than two.