My reason for following Saradomin is quite simple
"Evil must be destroyed in all of its forms, not mollycoddled and allowed to spread."
I believe Saradomin is the only god to really holds true to this stance, even Armadyl's actions during WE2 only occured after ages of being a soft touch. Saradomin actively seeks out and removes threats to his people, sometimes personally as seen in WE1, sometimes by enlisting the aid of his followers as seen in DoC. I disagree with things such as moral relativism because I believe they lead otherwise good people to inaction, and thus allow harm and suffering to befall innocents just because their tormentors have deluded themselves or don't know any better.
However, Saradomin does more than simply protect his followers and smite the wicked, he provides his followers with knowledge and guidance. With help from his teachings and organizations he has created, the human kingdoms have risen to become some of the most prosperous and secure places on Gielinor.
I am aware that my god has made mistakes, and will most likely make mistakes in the future. He has a temper and is prideful, but he also aware of this and doesn't hide the fact that he has made mistakes, as seen in DoC. He regrets his past actions, which is fitting, as regret begets wisdom and leads to self improvement. This also serves to remind us that he was once, and to some degree, still is, human. Indeed, humanity is another reason why I follow him over gods like Seren and Armadyl, as they are more connected to the elves and aviansie, with most of their followers being from those races. With Saradomin, the majority of his followers are human, or human-like, as are his clergy and greatest champions.
In short, I follow him because he is pragmatic, open about his faults, and ultimately, human.