Now, I haven't played .flow yet (although I do plan to) but one thing that occurred to me is that both Yume Nikki and .flow have roughly analogous "guardian" monsters that patrol the worlds and will, if provoked, attack you. In Yume Nikki's case, we have the Toriningen. In the case of .flow, the Kaibutsu. Both are pretty horrific, but the question is, who out of the two would win in a dreamscape fight?
On the one hand, the Kaibutsu seem more willing to kill. Several of them will brutally slaughter the protagonist (can't remember the name) such as in the Corrupted School event. The Toriningen on the other hand will never actually harm Madotsuki (either out of care for Madotsuki or some other motive).
On the other hand, the Toriningen are far more physically imposing, towering over most other NPCs. In addition, they cannot be killed. Ever. At all. Whereas (I think, I haven't played .flow yet so don't kill me) you can kill Kaibutsu, no matter how many times you stab the Toriningen, they will not die.
As far as I know, you also can't kill Kaibutsu.
But here is what I think:
Kaibutsu: Better attack and speed in cost of HP and some intelligence. They are vicious, but some of them hold weapons (the kaibutsu at the bar, anyone?) and some turn into Rust Demons, which are slightly if not even worse than the shown kaibutsu.
Toriningen are more "simpatic" creatures, hell, even when they are lunatic, they are still kinda funny (except on the manga adaptation). So they can count on a suprise element. However, I doubt that beaks, as well as its body size, can rack up damage. Remember that big size=/=danger. But we can at least count on better defenses.
Hmm. Interesting point there. The Toriningen are certainly a lot less "threatening" than the Kaibutsu. The Kaibutsu are quite obviously enemies, and something to be feared, whereas the Toriningen are seemingly friendly, harmless, even benevolent creatures (One thing regarding the Toriningen that are already berserk is that they are often found near or in the way of effects. Given what happens in the ending, it could be that they know what will happen if Madotsuki collects all the effects, and are trying to save her).
One thing that had occurred to me is the level of "power" the Toriningen and Kaibutsu have in their respective worlds. The Kaibutsu seem to be fairly simple minded denizens of the dream world, simply inhabiting it and at the mercy of the world and its dangers. Near the end, they are themselves killed by the Red Demons, and they seem to live in fear of Smile. The Toriningen on the other hand, seem to serve some kind of special purpose in the dream world. I imagine them as guardians and overseers of the dreamscape, protecting it and looking after it, ready to fend off or temporarily detain Madotsuki should she start destroying things or going places she shouldn't. They seem capable of strategic planning (such as the Toriningen that will appear in one of the igloos if Madotsuki stabs the one outside, and the little team of Toriningen protecting the Guillotine Room) and are a lot more intelligent than their .flow brethren.
Of course, like I say, I have yet to play .flow (I can't get it to work) but from what I have seen, the Kaibutsu are more like simple predators and beasts, whereas the Toriningen are closer to enforcers and guardians.
But hey, just my opinion.
Personally, I think the authors were going for two very different atmospheres of threats for both monsters. To briefly sum up my theories on both the monsters:
Toriningen - A representation of older, more stylish girls that may or may not bully Madotsuki, but certainly don't care for her. They're goofy looking, but strangely stylish; they wear modern clothing, they're tall and slender, they have cartoony features, but this is probably just referring to something like an overuse of makeup. And notice their posture! They're not slouched over, they're poised nicely upright, and their clothes accent an almost fashionable kind of walk with how they curve at the bottoms. Their existence is a reminder of futility; Madotsuki can never join them (The Toriningen picnic) and if she tries to beat them, they come after her and don't cause her pain, just mental torment. They scare her, and torment her. Like, they're malevolent.
The Kaibutsu, on the other hand, are supposed to be vicious. As you mentioned, they cause her physical pain, and most importantly, Sabi becomes one. This is the most important contrast with the Toriningen, I think; Sabi doesn't want to be like them, around them, or to even know of them, but she can't help it. They seem much more deserving of the title the fans gave them; they're monsters, in a very literal, straight forward sense.
The Kaibutsus' appearances interest me a lot more just because I'm more into morbidly creepy stuff, but I have to admit, I think the Toriningen had more thought put into their design, and are more unique. Like a lot of things in .flow, the Kaibutsu's main purpose is just to be scary, they're not really all that special. The idea of preppy, demented school children is… surprisingly common in Japanese games/anime. >_>