The Singapore flyer does get you up to a height to see a view of downtown but it's just a really really really slow ferris wheel and you spend a lot of time with a view of the workings of the wheel itself. You also have to share a confined space with other people's ill behaved children.
The cable cars, on the other hand, swoop over Mt Faber park, down over the water and then across Sentosa so you have quite a cool view the entire time. You don't have to share a car with other people so it's 150% more romantic and if an ill behaved child spoils your time it's your own kid and not one you adopted for 45 minutes. Also you can get a round trip ticket on the cable cars which makes it seem like a better bang for your buck.