I have both rulebooks... and well there are some changes... so if you want to have official tournaments... but for a normal game at home the ROTK book is of course quite sufficient.
The biggest changes... hmm there is a whole new section with River rules, siege rules and themed scenarios (or whatever they are called)
Personally i think of these the siege rules are the most important... but those can be found in TTT rulebook too, which you should be able to get significant cheaper.
The pros with the rulebook though is that:
1. Its in hardback
2. It has a quite detailed hobby section.
Cons:
No new themed scenarios (like the battle of the pellenor field, where they give suggestions what type of army to use ect.) but just the themed once.
So you will have to decide yourself.
Oh one last thing, you might want to consider buying it just for the satisfaction of being capable of talking in the tactical discussion Forum and to answer the questions in the official rules question.
And they are not bringing out new books for SBG every other month now no more