Yes, a federal flat sales tax would be better than an income tax, because a flat sales tax would be so much easier to understand. Also, people wouldn't have to have money come out of their checks each month, and they wouldn't have to come up with large sums of money each April.
An income tax discourages individuals from working by essentially being a tax on their labor. A flat tax means that individuals who are hard working are not punished nor treated as though they cost others more by the value of their labor. In short, if you tax labor, you will receive less of it.
Yes, a federal flat sales tax would be better than income tax. This would do to positive things. The first one is that there would no longer me a group of people rich or poor that feels they are being taxed unfairly. Everyone would be on the same playing field. With everyone on the same playing field, that would make political parties have one less subject to argue about.
Flat taxes penalize the poor and reward the wealthy. Progressive tax schemes, where, the tax duty is placed upon those most able to stomach it, the winners in other words is a much fairer system of taxation. When there is a flat tax the pain of taxation is most felt among the working poor.