Both have similar survival rates and are described as excruciatingly painful. Consider just the final augmentations and not the lifetime of training. You dont get the gear either.
Advantages of becoming a witcher: significantly longer lifespan, enhanced speed and strength, extremely enhanced senses, immune to disease and most toxins, very successful with the ladies.
Disadvantages of becoming a witcher: sterile, everyone can tell, vulnerable to pitchforks
Advantages of becoming a spartan: increased strength and speed, greatly improved durability, heightened intelligence and focus.
Disadvantages of becoming a spartan: become absolutely freakin massive, may have social issues