40 years of HDB con and swindle and still no one on this forum can answer this TS's question. Sinkies are damn pathetic, they deserved to get fucked by the PAP.
The answer to the TS question is that one is a form of ownership and the other is a long term rental agreement.
A leasehold 99 year private condo means that the improvements on the land (the whole building itself) belongs to the owners of the individual units but the land belongs to someone else. In singapore's case, some private condo land is owned by the govt. and some are owned by the various private land development companies. At the expiration of the 99 year lease, the land reverts back to the landowner, with or without the improvements, depending on how the lease is written. If the land reverts back without improvements, the owners of the units have to demolish the building and give the land back to the owner in the condition it was first leased out. This will all be negotiated in the lease. The advantage with the leasehold is that owners actually own the physical unit they live in. If all the owners decide to do a self-enbloc, they can in theory tear the building down and build a higher density (taller) building with more units and sell it. The new buyers will have the balance of the 99 years original lease.
For a HDB flat, the 99 years refer to lease agreement, not a leasehold. Its no different that if you lease a car for 5 years. You don't own it, you are just renting it for the long term. Except in the case of the HDB lease, the HDB requires you to prepay for rent 99 years in advance. normally when you rent a flat, it is usually for a 1 year term and each month's rent is paid when its due. You would never agree to pay a flat rental 1 year in advance, yet people willingly pay for their HDB flat 99 years in advance.
The difference lies in other areas too. For example, HDB has no intention of honouring their 99 year lease. The quality of the construction does not allow it to last for 99 years. Also, HDB has so many clauses in their lease agreement that they can easily move you out before 99 years on the pretext that they are tearing down your block and move you into more expensive newer "upgraded" flats through SERS. This is usually not possible in a leasehold private condo because the law is quite specific on this part, and since the landowner does not own the building, he cannot force the owners to move out and tear it down for a higher density building.
I hope this clears up the TS's question.
In this case HDB is better, since they keep "upgrading you" it becomes as good as freehold isn't it? And who says construction quality is no good? These days materials used for construction of HDB go through much more stringent QC checks than private property.