Where to actually place it
On a resume, the footer or header corner works better than the middle of the page — it should read as supporting information, not compete with your experience section for attention. On a business card, the back is almost always the right call; the front is for your name and contact details, not a black-and-white grid.
If you're handing out both a resume and a business card at the same event, it's fine — encouraged, even — to use the same code on both. Consistency reads as intentional, not repetitive.
What to actually link to
Link to the single strongest thing you have, not your homepage by default. If your portfolio homepage is just a hub with three more clicks to any real work, consider linking directly to your best case study or project page instead — you can always add a 'see more work' link back to the hub from there.
For a resume specifically, a common pattern that works well: one QR code to a portfolio site, and if you don't have one, a QR code straight to a PDF one-pager of your best 2–3 projects hosted anywhere public (Google Drive, a personal site, even a well-formatted GitHub Pages link).
Keeping it looking professional, not gimmicky
Match your accent color if you have one — a QR code in your resume's existing accent blue reads as designed, while default black on a colorful resume can look like an unstyled afterthought. Keep it modest in size: on a business card, roughly 1.5–2 cm square is plenty for a phone scan at arm's length.
Test-scan it yourself on both an iPhone and an Android phone before printing a batch of cards. It takes ten seconds and catches the rare case where a color choice reduces contrast below what a camera can reliably read.