I agree, if it's an option for you, do both! I started working with a trainer last year in conjunction with my Cathe workouts and I've learned so much and have had great results. You don't need to work with them 3 times a week, I did once a week sessions for six weeks and now I only make an appointment when I want to change up my program - it's more of a give and take now too, we bounce ideas off of each other to come up with something new that's appropriate to my goals.
I'm with Shonie on the heavy lifting - it's made all the difference.
As far as your trainer's caliber goes, it depends on your gym in a lot of ways. My gym only hires trainers that have degrees in their field together with certain types of certifications. As far as nutrition, I know that the trainers at my gym only give general advice, no specifics. They have a nutritionist on staff they refer you to. I've had great success with Leanness Lifestyle in changing my eating habits.
Anyway, I split my time between Cathe and the gym and I love it this way.
ttyl,
Randi