You're both right. The bulk of enforcement actions, on the basis of volume of actions, is against low-income individuals because they are the most likely to underreport income (especially under-the-table compensation received as cash).
However, the bulk of enforcement penalties is against the wealthy, because they are the most likely to engage in large-scale tax evasion. Though within this category, it's actually the mid-tier wealthy, generally small business owners, who are most penalized, as they are the most aggressive in claiming deductions and credits to which they are not actually entitled. The ultra-wealthy will claim whatever deductions/credits their tax advisors say they can, but it's not worth it to them to be aggressive since the savings is usually insignificant compared to the potential penalties, or the cost of litigating a tax action.
However, the bulk of enforcement penalties is against the wealthy, because they are the most likely to engage in large-scale tax evasion. Though within this category, it's actually the mid-tier wealthy, generally small business owners, who are most penalized, as they are the most aggressive in claiming deductions and credits to which they are not actually entitled. The ultra-wealthy will claim whatever deductions/credits their tax advisors say they can, but it's not worth it to them to be aggressive since the savings is usually insignificant compared to the potential penalties, or the cost of litigating a tax action.