Building momentum takes time and energy. Once you build it though, it carries you with much lower effort.
There are elements of momentum which are exciting and truly useful. The excitement comes from the constantly increasing pace - something new every day - faster, better, higher, stronger. The efficiency comes from the inverse relationship with effort. Hard to build up, once built it is much easier to sustain. And so as you progress you get into the sweet zone where effort decreases and reward increases.
Rhythm is something else again. At it's core it is about repetition which sounds monotonous - but it's really about getting into a groove where things fall into a natural cycle, where highs and low even out, where night follows day, follows night, follows day with a natural inevitable, satisfying constancy.
Both are interesting. On balance though, I'd take rhythm over momentum.
I think the natural world is more about rhythm than momentum. From phases of the moon, to cycles of weather, most natural phenomena come and go in a pattern. What goes up, inevitably comes down. So while bursts of momentum are useful and exciting - in the end we will always need to find a sustainable rhythm.