In light of the new SimplyGo, should I use my DBS Visa Black Card that has a higher miles earn rate, or my Citi PremierMiles Card which has no expiry for miles, but a lower earn rate?

I think it depends on how fast you will be using the miles. In my opinion i will use Citi PM card.

DBS points earned via the DBS Visa Black Card expire within a year of earning so if you can hit the minimum transfer block (5,000 points) within a year, go for it. Otherwise, I would stick to Citi PM card as it is has no expiry so I can plan longer and not be rushed by 1 year expiry date.

This would depend on when you wish to spend the miles. A good rule of thumb is to look at when you wish to spend your miles. If you have/are going to buy big ticket items and subsquently will redeem the miles, then it would be wise to use the higher mile earning rate DBS Visa Black Card. On the other hand, if there is no foreseeable big spending/redemption, then it would be a beter choice to utilize the Citi PremierMiles Card instead.