A sling is a very dangerous option - even in a relatively mild crash (say, your taxi fails to brake in time and slams into the back of a stationary vehicle) at a relatively low speed (10-20 km/h), the force of your body moving forward could cause serious injury to the baby (G-force will increase your body weight exponentially). I was in a taxi once that failed to brake in time and slammed into a stationary car at a medium speed (approx 30-40km/h). Luckily I was wearing a seatbelt. But my body jackknifed forward (and if I'd been wearing a sling I'd have crushed the baby to death) and then slammed back. I was in physio for serious whiplash injuries for months and still feel the effects 10yrs later.
People justify slings by saying HK taxis don't go too fast (as there aren't many expressways), but the speed of the crash only needs to be about 30km/h for serious injury to occur.
Also people say the journeys are short (therefore less likelihood of a crash). I've heard that statistics show that the majority of crashes occur within the first 5 minutes of a drive.
Honestly, this is the life of your newborn you're talking about. Just get a Maxi-Cosi Cabrio or similar seat that attaches on to various models of wheels (eg, Bugaboo, Quinny etc). Strap it in to the taxi (which takes about 20-30seconds). And keep your child safe.
If cost is an issue, buy secondhand from a reliable source (or buy new and sell on afterwards so you don't end up paying too much in the long run).
Congrats on your impending arrival, btw.