Yeah... researching multiple models of missile (or engine) isn't a big problem, because Practicals typically cost only a few percent as much as a Theory tech does.Is waiting until we can improve the blast effect without giving up the retargeting a bad idea?
Otherwise I’d research both and decide later, as our situation may change (enemies discovered!), and the advantage of one particular choice may be clearer.
The "three stage thermonuclear bomb" tech was 30,000 research points... but the new missile design would only run about 600 or 700. A few percent.