I say Giga's! They have wayyyy cooler animals like Micro Chimp, Baby T-Rex, Computer Koala etc. Not just the same ole' Kitty, Doggie. But it's my opinion that even the cat and dog versions are better in the Giga Pets. Yes, they're a tad harder, but much more realistic than Nanos because they simulate a pet much better.
Also, I second the person who said they have better graphics and are "cuter" very true. They have a more appealing casing as well.