Are you sure your income precludes regular Roth IRA contributions? If so, I'd go taxable, if you can't roll her IRA into a 401K or similar and do the backdoor Roth.
IRA Pro is that your growth will be tax deferred.
Potential IRA Con is that eventually that growth will be taxed at ordinary income rates. Capital gains and dividend rates are currently lower than ordinary income rates. Another potential con is the record keeping you'll have to do to avoid paying taxes twice on the contribution amounts.