Constant terms at each level of a recursion tree
For each specific level $i$, it is true that the cost is $(3/2)^i n - O(1)$. However it would be misleading to state this in general. Suppose for example that the cost at level $i$ were $(3/2)^i n + 2^{2^i}$, and that there were $\log n$ levels. The dominant factor in the total cost would be the &quot;constant&quot; term.
However, in your case, you can say that the cost at level $i$ is at most $(3/2)^i$, which gives you the required upper bound.