The cheaper you go, the less accurate you can expect the measurements to be, that's how i look at it anyways.
API's are good test kits for a decent price. I use them myself. Only thing is.. i'm not too sure, but i think you can use the Freshwater test kit to test those basic things like Ammonia, Nitrate, and Nitrites, but i think the Freshwater version has pH and not high-range pH which is what you want for saltwater, someone correct me if i'm wrong. I used a freshwater API test kit back when i had my 55G and everything tested correctly. I compared my results to a brand new saltwater API test kit and all tests came back the same so i'm pretty sure you can use for freshwater test kits.