This is a very good question. Earlier case law (e.g. Sansone v. United States, 380 U.S. 343, 354 (1965)) said these were two, distinct crimes, but several Circuits have more recently treated these as constituting basically the same offense. These Circuits maintain that they both consist of an Internal Revenue Code Section 7201 offense of tax evasion. Thus, “Section 7201 evasion,” as it is sometimes called, may be committed by either attempting to evade a tax assessment or by attempting to evade its payment. See, e.g. United States v. Mal, 942 F.2d 682, 686 (9th Cir. 1991), where the Ninth Circuit held that “§ 7201 proscribes the single crime of tax evasion, a crime which can be committed either by evading the assessment or the payment of taxes.”