You know, I was told not to use a harness with a bully because they will drag you! Instead, we do VERY strict heeling and loose leash walking practice. Matilda still pulls sometimes - but not to the point of choking or anything like that. She'll just pull the leash until it's taut and then stops.
I definitely think a harness is better if you have no way of controlling the dog, but I agree with training being the best option. It does take a lot of time, though.
I've tried using a harness on Tilly before (her car seatbelt is a harness, sometimes I just attach it to the leash if we're about to get back in the car), and she doesn't like walking on it. I've been told it is also confusing to the dog sometimes, because they are pulled from their entire upper body, instead of a very clear direction (like on a collar). But again, this is assuming they're well trained and not pulling back. I think it just depends on the personality of your dog.
Also, something really cool I saw recently is a leash that is made out of a spring - like a bungee cord. You would definitely want it on a harness just for neck safety, but basically, the dog pulls, and the cord pulls them back - without extra effort on your part. Could be a great option as opposed to a slip collar! The more they pull. the quicker they come back to you!