Maybe I'm attributing too much strategising to Sunak but I think either of the 2 will deliver the disastrous tax cuts, just that Sunak is thinking ahead to the next General Election whereas Truss is thinking about her immediate personal greasy pole ascent.
Next election is going to be dire for the Conservatives under most scenarios. Sunak (or somebody thinking strategically) might think that for the wider electorate (mainly the Conservative "inclined" but not Party members), tax cuts now will be well forgotten in 2 years with massive inflation, cost-of-living crisis, etc. But hold back those tax cuts until just before the General Election and he'll sway many of those Conservative inclined votes (basically buying votes with the public purse).
But Truss wasting the tax cuts not might get her the votes from Conservative Party members but after 2 years they'll be long forgotten and those conservative inclined will have been enduring 2 years massive inflation, intrest rate rises, cost of living, heating massive homes, etc. so the General Election will be a Conservative disaster.
In many ways, Sunak may realise that getting elected by using tax cuts now gives him 2 years before a disaster of an election. But lose now telling the "responsible on finances" story gives him a 2nd go in 2 years when he'll be starting from a real low point where "the only way is up".
Or maybe Sunak is just trying to find a different story from Truss' "I'm Maggie, I'll give you everything" manifesto".
Ian