There are key differences. Houdini is almost entirely parametric, so most modelling and anything else can be done via nodes, which is as advantageous to some situations as it is disadvantageous in others. But it has ultra-powerful particle, simulation, smoke, fire , water and ocean sims - and huge complex mograph-like systems, and all eminently user-codable - you name it - all can be done there, and it is ludicrously cheap for the power it offers.
So while, yes, there is some crossover in functionality, and they are both DCCs, they are not really in direct competition with each other - Cinema is not trying to be fully parametric, and shouldn't do, because some tasks, for example modelling, are 100 times faster with traditional pipelines. Imagine having to add and setup a (sometimes quite complex) node for every single stage of modelling a form ! There is room in a workflow for both apps. To use a blunt little tool of analogy; If Cinema is like hand-painting a beautiful picture in a nicely lit studio, Houdini is like trying to paint a hallway through a letterbox - ie a LOT harder initially, but capable of great great things once mastery is achieved and you get used to its way of working... if you have both, a TPR and some decent post, you are pretty much ready for film VFX.
Personally I tend to fall asleep while I am learning Houdini, so I remain largely Cinema-centric where I already know most of the things, and modelling is all nice and familiar and no more parametric than I want it to be...
CBR