I'm SIN-based and use both SQ and UA programmes.

I credit all SQ and MI flights to Krisflyer, and nothing else (apart from credit card points):
- not sure if there are good options for credit card earning rates in SG (just haven't done any research)
- if you fly MI every now and then, obviously Krisflyer is pretty much the only option
- I find it pretty challenging to use KF miles; typically would like to redeem for long-haul J, booking max. 2 months out, but this is difficult (I don't mind the surcharges, but availability is an issue)
- On the other hand, would not consider using my hard-earned Krisflyer miles flying anything else than SQ in premium classes

UA
- I tend to fly a fair amount of flights on other Star partners, and find UA convenient for this
- Visit the US a few times a year, so with some planning I can get the UA segments together
- I don't care about status with UA, and have Platinum (previously 1K)
- Prefer to use miles to fly long-haul J, booking close to departure (I find united.com pretty convenient for these type of redemptions)

I would probably stick with SQ in your case if you get miles via the credit card. On the other hand, I find UA pretty good as well. Wouldn't worry about status too much.