check your information
Be careful when taking advice...
Trainers and instructors may not be qualified to give any better information than your average gym rat. Sorry, but it's true. Anyone can read an article, (in fact anyone can write an article) that doesn't mean the info is based on science, sound, reliable or accurate.
Qualified people will give their credentials and cite the sources they got their information. They will usually want you to follow up and become better informed yourself.
They would also ask for further information before answering your question about cardio or weights first - like: what is your goal? how long are your cardio sessions? what is your fitness level? how heavy are your weights? reps and sets.... etc.