They're both good. It really depends on what you like in an AC. The Kodiak is a new, powerful bird. It is great at altitude, good at getting into tight places. It's kind of a hotrod in the bush aviation world. I confess I find it lousy for GA flights, though. It just isn't that kind of plane. The G1000 eats my FPS, as does the cockpit sound... I've got my workaround, but it may give you some grief at first.
The Carenado planes, especially the latest ones, are drop-dead beautiful. They are not fancy as far as features, but they deliver great, reliable performance. I've never been disappointed by a Carenado purchase. The C337 is an unlimited poly model, meaning that it breaks the limit set for normal FS9 models and has just as many polys as the FSX version of the plane does. That aslo means that it is hard on frames, though the smoothness of the modelled 3d gauges offsets that a little.
If you are only buying your first payware now, I'd suggest buying Freight Dogs by Aerosoft. That is a little older, true, but it's got some of everything. It's kind of a sampler of various types of payware addons to give you an idea what you can expect from payware and let you know what you want to spend your money on. It's got planes, scenery, addon modules, and enough environment to show you what environmental upgrades can do. It was my first payware and I stil think it was one of the best values I've ever gotten.