I have both a Giga Pet and a Littlest Pet Shop pet. My opinions:
Giga Pet: Too big to be of any convenience carrying around. It's a very nice virtual pet, and I'm saying this despite greatly disliking money-based virtual pets. It's not bogged down with games or other things to do, you just care for it. Some nice features, like once it connects with someone, it might start to miss them, or how you can change the volume and screen brightness (this I love). That said...I greatly dislike money-based pets. I find I almost constantly have to make money digging as its metres are hard to put up (particularly the Happy one).
Littlest Pet Shop: It fails at being a virtual pet. From the name I almost knew it would, so I expected it to.
I was actually kinda pleased with this one. It's very nice for traveling. I didn't find I needed the map after a while, it has a (very limited) built-in map, too.
However, once you collect all the stamps, mail is the most annoying thing on Earth.
I've never played with a Mori/Umi no, so I wouldn't really know how going about using the sound sensitivity works, but wouldn't it be a little strange shouting at your Tama in public? (Then again, I do that already...x_x)
LOL that's why tapping also works

Also: the oldschool gigapets are better than these bulky new ones. I think Tiger Electronics needs to go back to the basics.