Yep, lots of options which has to be good for competition, and you have two different views already in the previous posts.

In general, if you book in advance/online you will get a better price than if you just bowl up - in doing the latter, you are "committing" yourself to that particular form of travel, come what may, and the travel firms know this. However, if you go to Dover, rather than the train terminal, there are a number of options, so you can shop around for a deal: all depends on if you want to dedicate time and effort to this type of thing - I have done a bit of "people watching" in the past when this way of getting a ticket was very common, including the option of taking a return ticket and ditching the return portion.
Very amusing to see.
Be careful about day return tickets: I think some operators don't sell them after certain times of day (I've ended up paying full price!!). Pay cash, it cannot be "traced": even credit cards have addresses related to them and the ferry companies keep records on their computers.

I have never used Norfolk Lines, but I have heard good reports about them: I think that they don't take coach parties, so you get away from the school trip syndrome - hordes of kids, running riot around the boat while the teachers disappear to the bar. A complete PITA at some times of year.

Look back in the posts in here and you will find info about Fastferries - they stopped taking bikes for a while but they are now making the effort to bring in our custom for their service out of Dover.

The train: good, fast, efficient - you don't feel as if you have really stopped, which can be a disadvantage if you are in need of a break. But, they do have good services at each end (none on the train), so there is no real reason why you can't stop "properly" - but, psychologically, you just want to press-on.

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Dave