Thanks CR001. Q13 does explain that PBS dependent extension is better.
See 3b below.
Dependants of PBS (Tier 1 General, Tier 2) migrants
Spouse / Partner
1. The dependant is subject to qualifying residential period to be eligible for settlement. For the duration of qualifying residential period and associated conditions that may apply, see Note below.
2. If the dependant has completed the relevant qualifying residential period at a time that the principal migrant applies for settlement and meets the other requirements then the dependant may be included in the principal migrant's settlement application.
3. If the dependant has not completed the qualified qualifying residential period at the time that the principal PBS migrant applies for settlement, then:
a) If the PBS dependant's current leave will expire after he/she has completed the qualifying period and the dependant meets the other requirements for settlement then the dependant may apply for settlement directly (319E).
b) If the PBS dependant's current leave will expire before he/she completes the qualifying period then the dependant can apply , before his/her current leave expires, to extend his/her PBS dependant leave until such time that he/she becomes eligible for settlement (319C and 319D). Though switching to FLR(M) leave remains an option but extending PBS dependant leave is simpler (e.g. - only maintenance funds to be shown, no English language requirement).