Regardless of which is cheaper right now, you don't want a monopoly. In the long term, having 2 or 3 profitable suppliers is much cheaper than having one cheaper-for-the-moment supplier. There are several reasons for this:
* A monopoly supplier could raise their prices and stop innovating. (See: ULA). It's incredibly difficult, slow and expensive for a new supplier to get into the space launch business.
* If there's only a single family of rocket being used, and they have a rocket explode so they ground their fleet, you're stuck. We know such failures happen, even Shuttle had a ~1.5% failure rate. If there are several different models from different suppliers, then common faults are much less likely.
* If your sole supplier goes bust / has its workers strike / has its facility wiped out by a natural disaster, you're stuck. Multiple suppliers means you have less single-points-of-failure.
That's why NASA were careful to award ISS supply contracts to two different companies (SpaceX and Orbital), while the US government also has launch contracts with a third domestic company (ULA). That gives the US government three local suppliers to compete for its missions.