It's going to be a combination of airline and the specific aircraft employed, coloured by any status you might have in frequent flyer programmes.

If you fly Y on company policy, then perhaps fare is not so important - so why not stick with SQ?

I favour SQ in general on Singapore routes, but they have some very crap economy cabins within the fleet, and tickets can be expensive and the mileage rewards insignificant on low fares. I'd prefer Swiss any day (they just click with me), but they don't often fit well with budget travel. I'm happy to take connecting flights where these are cheaper, usually Turkish or Malaysia. I generally avoid US carriers in Y.

I'm kind of tied to STAR and Oneworld carriers because of the benefits I get, so I miss out on Aeroflot and Alitalia

For hard information on seat pitch and hints on "best seats" Seatguru et al can be helpful.