Clarification on flight seat upgrade method?

Hey all. Just got my Venture X card recently and looking to book a flight. I want to book with the portal cause a main point of this card are the point multipliers in my opinion. Whats the general consensus on using the portal specifically with paying for seats? Should I:

Book all through travel portal and just confirm reservation is accurate directly on airlines site?

or

Book basic economy through Cap 1 portal then go direct to airline site and pay for seat upgrades?

All input is appreciated!