I think Jade and Beck, because they are (well they was) dating for three years, and although they are polar opposites have shown that they Amore despite all problems and differences in their relationship. This explains better in the song "wouldn't change a thing"
I like Beck and Jade more, personally. Couple wise I think they're pretty much on the same level. There are good and bad things that come with every relationship.
I think that we need to remember that Beck can be controlling, easily embarrassed, and full of himself. I really don't think this would be good for Cat.
To be honest, I am interested to see Cat in any relationship right now, because even though Jade is harsh and manipulative, she acts like an adult... Cat doesn't. I don't know how Cat can really be in any sort of relationship without a changing her whole, "I'm a complete moron." thing she has going on.
I think Beck and Jade used to be really cute and if we got back to how they used to be I think they're almost perfect together. (Sorry if this answer is weird, I'm distracted watching friends)
I definitely think Cat and Beck would make a better couple. They "fit" better than Jade and Beck, simply because of personality. I never really understood how Beck and Jade got together in the first place... It's kind of like when te suddenly realize, "How DID Trina get into Hollywood Arts?" Cat and Beck would hit it off better and have fewer disputes, IMO.