Either is available to a Canadian citizen. Advance filing gives a written decision before she resigns; a port-of-entry application is faster when the documents are complete and the licence question is settled.
Choose the route by how much certainty the resignation needs
Canadian citizens may present a TN application at a port of entry or a preclearance location and seek a decision during inspection, or the employer may file Form I-129 with USCIS and the pharmacist may travel after approval. The choice is about risk tolerance rather than eligibility. A pharmacist leaving a hospital position in Mount Pearl on notice may prefer an approval notice in hand before the last shift, particularly if the state licensing pathway is unusual and the officer might have questions. A pharmacist whose licence, offer and degree are straightforward may prefer the port-of-entry route and avoid the advance-filing wait. In either case, the hospital's start date should be written as conditional on admission, and any relocation benefit should not be triggered by a booked flight. Depending on the reason for a port-of-entry refusal, she may be able to correct the file and reapply; the refusal is recorded and should be disclosed as required.
Hypothetical example: a Canadian social worker has a signed offer but the employer is still revising the duty description after a community-services contract changed. The practical decision is to delay travel until the support letter describes the final role. The executed letter settles what work is being requested, while a later email cannot reliably repair a material mismatch. If the employer wants advance adjudication, it can evaluate an I-129 filing; otherwise the applicant should carry a complete, current package when seeking admission.