The other thought here is selecting a single branch and staking it to get a tree form in later years; there is a row of these plants in the town where I work that are 5' tall trunks that are 10"-12" in caliper (easily a hundred years old!). They are cut back to the same point yearly and the lady who owns the house sells the lot to a florist at a tidy profit. More importantly these trunks don't bend (for the obvious reasons) and make for a very nice presentation...
That bigger lower branch sounds like a good start to a tree form; this plant can be pruned right until June really and not lose the blossom, so I would cut it to the selected trunk and leave the whole leader for now (minus blooms and really soft wood). Wait for the breaks in spring and select your branches, cutting them back about halfway in May. Wait for fall and blossoms; repeat (until tree trunk reaches desired height. That's my favorite way to show off this fall beauty!
HG