My opinion is that it doesn't matter which processor you buy because you will have the same important dependencies irregardless. Intel or AMD, you will depend on the expertise of the operating system designers, the chip manufacturer, and the trustworthiness of the code you run. The presence of disclosed vulnerabilities in Intel chips does not reduce the probability of undisclosed or undiscovered vulnerabilities in AMD chips. The number of possible vulnerabilities in AMD chips is a very large number. The number of possible vulnerabilities in Intel chips is a similarly large number.
Basically the choice between Intel and AMD in the context of in-chip vulnerabilities comes down to whether one feels better choosing a chip with known vulnerabilities + significant effort to mitigate or a chip without known vulnerabilities and without significant efforts at mitigation. In both cases, unknown vulnerabilities are probably equal.
To put it another way, building a computer is largely a consumer process not a technical one. The biggest security risk is software you download and run, not hardware flaws (e.g. rowhammer)