Low poly to high poly, or high poly to low poly? Which is easiest to do&mdash;make a high poly model and then turn it into a low poly one? Or the other way around?
I'ts going to very much depend upon the model. In some cases, adding geometry will be easier, in other cases, removing geometry will be easier. There's not really a "one size fits all" answer to the question. If you're asking about a specific model, edit your question, and provide links to images or blend files.