I know they aren't really comparable, I'd just rather use Kadabra if I'm going for a Psychic type on my team. Besides, Kadabra makes up for its terrible defenses with immunities to weather, hazards, and any other damage that isn't a direct attack. The differences between the two are bulk, attack method, and how they deal with status. I prefer Kadabra simply because it's powerful without setting up.
Really it just comes down to what people need. If you need a bulky set-up sweeper, Musharna is pretty good, although can be walled by Dark types. If you'd rather have something that can hit hard right out of the gate, Kadabra is fast and pretty strong, it's just not going to take a neutral hit unless it's behind a sub.