Good question. Umbridge creeps me out more because people like that can get away with abusing their power far too often. But I wouldn't say the issue with Voldemort is that he's "unrealistic" as such - sociopaths are unfortunately quite real. They're just less common than dangerous zealots.
Umbridge is like a really stinky cheese. Good in certain doses, but had she been around longer I don't think the love/hate would be there, it would be just the hate/hate which gets the book thrown in the recycle bin. Voldemort was more tolerable for longer periods of time.
I loved Voldemort as a villain, but there's no doubt that Umbridge was the most hated of the "bad guys".
And even if she was unpleasant and twisted, I don't see her as a potential leader of an extreme group, like Voldemort...
Also, how is she more 'realistic' than Voldemort? They're both magical, so..
I think that as you get older, Umbridge gradually has more of an effect than Voldemort, because you come to fear different things. When you're a kid, a more fairy tale-like villain, such as Voldemort, who is almost all-powerful, and seems to be everywhere, and who can never die, seems much scarier, but as you grow up, you start to realize that Umbridge is much more real, and that makes her scarier. Because there aren't a lot of near-immortal wizards out there, but we all know that there are people who are only truly happy when hurting and humiliating everyone around them, who would do just about anything for power, and who will hide all their cruelty behind saccharine smiles.
I don't know about "better"...they're too different kinds of villains. Two different planes. They're both really good characters and really good villains. The awful thing about Umbridge is that you know people like that exist, and you've probably seen them, and she's a member of the government and you can't do a goddamn thing about it. Voldemort, though, is purely made of magic and isn't human at all...so he's kind of less...well, I don't want to say less "scary" because that's not quite true, but you know what I mean?
It's just kind of tough to compare the two because they're so different.