The verdict: Digi's price is lower because its coverage is smaller.
Sometimes, I do not even have a signal at my own home when Maxis's signal strength is at full bar.
Another verdict: Digi's price is lower because its data speed is slower.
Digi's do not have HSDPA but Maxis does, but when I do get a Digi's 3G, I think the speed is sufficient for my iPhone.