Fast tier decision: access is not the upgrade
Titanium is the lower-commitment starting point if you simply want access to World Tours and Resort Getaways; the official enrollment display places both features in that tier.
Platinum presents a different question: its stated delta is 120 more Travel Points per month plus Lowest Rate Buy-Down. A label cannot answer whether that changes the cost or terms of your trip.
Without a named trip, Titanium is the honest default. A documented booking can later show whether Platinum deserves another look.