It is more F-15s AND F-35s! The decision to shift PLAB already funds both acquisitions. The F-15SE is not as stealthy as the F-35 so the incremental benefit compared to the F-15SG may not justify the added cost i.e. F-15SG almost as effective. AFM july 2013 issue already reports Singapore has ordered another sqn of F-15SG as F-15SE & F-35 not ready yet. In any case, many of the F-15SE features e.g. CWB can be retrofitted on F-15SG.
If just more F-15SEs are bought, Singapore will need more airbases as each F-15 needs minimum 8000 ft runway.This has to be funded by tax increases as PLAB cannot be spared. Saudi spent $30 billion for 84 F-15SA (not even -SE) so that's a lot of taxes to raise.
On the F-35, recent reports indicate 75 will be bought which presumably will replace the F-16s. In selecting the F-35B variant, this allows the RSAF to buy more fighters in exchange for PLAB land sales. The F-35B can take off in under 600 ft of runway without ski jump and no need runway to land so more aircraft can be based per runway! Recent land sales data show each hectare can fetch as much as S$300m or more for Paya Lebar area (todays prices not even counting future inflation). In 2030, the land price could be even higher. PLAB = 800 hectares.