Short answer. Resistance is the lesser offence: refusing to submit to, or seriously disobeying, an officer performing official duties. Direct assault is the graver one, involving attack, force, intimidation or serious resistance against a person in authority or his agent. The resistance article applies only to conduct not already covered by the assault provisions.
What the law says
shall be imposed upon any person who not being included in the provisions of the preceding articles shall resist or seriously disobey any person in authority, or the agents of such person, while engaged in the performance of official duties
Revised Penal Code, Article 151 — Resistance And Disobedience To Authority. Read the full provision →
The residual clause is the whole distinction
The resistance article opens with a limitation that decides most cases: it punishes a person not being included in the provisions of the preceding articles. Those preceding articles are the ones on assault upon persons in authority. So the two offences are not really parallel choices; they are arranged in order of gravity. Prosecutors look first at whether the conduct amounts to assault — an attack, employment of force, serious intimidation or serious resistance against a person in authority or an agent engaged in official duties. Only if it falls short of that does the resistance article take over. A charge of resistance is, in effect, a finding that the graver provision does not fit.
Two grades within resistance itself
The article contains its own internal ladder. The first grade covers anyone who shall resist or seriously disobey any person in authority, or the agents of such person, while engaged in the performance of official duties, and carries arresto mayor with a fine. The second, lighter grade applies where the disobedience to an agent of a person in authority is not of a serious nature, and carries only arresto menor or a smaller fine. Two things follow. Seriousness is an element, not a sentencing detail. And the lighter grade is written only for disobedience to an agent, not to a person in authority himself.
What both offences require of the officer
Neither offence is committed against a uniform in the abstract. The officer must be a person in authority or an agent of such a person, and must be engaged in the performance of official duties at the time. An officer acting purely in a private capacity, on a private errand, or outside the scope of his function does not bring the provision into play, and a scuffle with him is judged like any other altercation. The status of the officer as authority or as mere agent also matters, since the lighter penalty is available only for non-serious disobedience to an agent. These are factual questions, and they are frequently where a defence is actually built.
About the peso figures
The amounts above — a fine not exceeding ₱100,000, and the ₱2,000 to ₱20,000 band for non-serious disobedience — are the current figures, as amended by Republic Act No. 10951 (2017), which revised fines and property values throughout the Revised Penal Code. Many reproductions of the Code still in circulation, online and in print, print the original 1930 amounts, which are drastically lower. If a source quotes you a different fine for this offence, check which version it is reproducing before relying on it. Note also that the penalties here are the statutory range only; the sentence actually imposed depends on rules of application this article does not contain. If you are facing a charge of resistance or assault, book a consultation rather than assuming the lighter provision applies.