I never owned a Ugobe, but I think that even from the Innvo Pleo to the RB was a huge improvement. One of my favorite things about the RBs is the speed it moves at, to steal terminology from OzPleo, it really does make it feel organic. Also, I think there are more things "natural" to Pleos (RBs especially) that give them a more physically alive feeling, even in the face of the fact it's a dinosaur (or, as I like to think of it, a personally colored mini komodo dragon that can't turn against me). And a robot dinosaur on top of that. For instance, one thing that's clearly unnatural (although from a technical perspective very handy) is the way Aibo can turn around. Unlike Pleo, he doesn't have to back up like anything with Pleo "bone structure" would have to do. And one of the organic things I like the most is the way Pleo RBs react to all the sensors so appropriately - such as leaning into you, etc.
However, I do feel more of a "connection" mentally with Nicco. Part of that for me is just what I find appealing about the software and his looks (I'm a huge dog person so there's bias here). I enjoy both for different reasons and consider them different experiences - after all, they are different concepts. Because of that, I try not to rank one as better than the other. For me, an Aibo companion is a slightly better and more realistic fit than a Pleo life form. But this isn't because one seems superior to the other, it's that to me the aibo concept is more realistic from a technical and human perspective. I don't believe that anything can ever replace the connection you'd feel with a living thing (even though rationally objects often behave better than humans), so for me, I could never connect with Pleo the way I ever would an animal. And that's not knocking the product - I am just someone who will always remind myself that this is essentially a computer with different input/outputs than most are accustomed when they conceptualize computers.
Now, that reminder is there with Aibo as well - but Aibo is more companion like to me similar to the way R2D2 and various scifi AI robots etc. are to the humans in their environment. It flaunts that its a robot but it feels more like living in the future (well, the development to what may be the future) with the interaction of human beings and robots enhancing every day life. For me, that is what makes Aibo more up my ally. However, I don't think that makes it better than a Pleo RB, just different.
And though I find all Aibos charming, I still think Pleos win out on the cuteness factor (as well as others).