*A has a better network coverage in asia than one world, although neither have anything within China. Shanghai Airlines is joining *A soon, as is Air China. Meanwhile Dragon Air is tied up with One World so potentially may be a future member.
Flying SQ on the cheapest fares won't get much as their most discounted fares are non mileage earning on any scheme.
As well as PPS (for first/business pax), SQ does also have the traditional silver and gold tiers. Gold is 50k miles (on your travel patterns easier to attain than OW equivalent Emerald). However if you fly Thai a bit, they may be a better program since can get 2 year status and they are freer with upgrades (plus Thai fares tend to be quite a bit cheaper than SQ).
If you are likely to travel within europe a lot on the alliance of your choice, then that is a further reason to go for *A IMHO. With one world the hubs are at the edges of Europe (LHR, HEL, MAD) and so a lot of european travel involves a choice between outside alliance or much backtracking. Whereas *A hubs are in the middle of europe (FRA, MUC, ZRH, GVA, VIE, WAW, BCN, LHR, CPH, OSL, ARN, MXP, FCO, HEL, ZAG, LJU) making it much easier to get around.