Thanks.
They follow very similar bonds but have slightly different indices they match against. USHY uses a 'broad' US high yield index, XHY a more generalized high yield index. But performance (currency adjusted) has been fairly close over five years but USHY has small advantage in performance. USHY is far larger ($25B vs $885M) and cheaper (fees 0.08% vs 0.56%). High yield bonds should do well with lower rates, UNLESS there is big concern about a recession and corporate credit quality. We would be comfortable investing in either but USHY looks a bit better.
Authors of this answer, directors, partners and/or officers of 5i Research and/or affiliated companies have a financial or other interest in XHY.