The butterfly on the right is a Monarch. Monarchs do not have the line on their bottom wings and the dots are not in sets of two, they are more scattered.
Monarchs are larger than Viceroys. They also fly differently. The Monarch glides while the Viceroy flies sort of crazily.
Lastily, Monarchs fly to Mexico in the fall. Viceroys overwinter in their leaf blanketed chrysalis and emerge in early spring. Monarchs will back and arrive later in spring. If they are blown off track, they can find their way back.
It is hard to tell the difference. It is fun to learn enough to tell which is which.