Very recently, an article from Bangkok Post broke the news that China has own the competition to supply 3 submarines to Bangkok Navy. While this is China's third export deal of conventional submarine in the past year, this is the most significant one in terms of the competitiveness of the competition. Since China and Thailand has a history of military transactions, this deal is unlikely to encounter the kind of scrutiny like the Turkey long range SAM contract.
At present time, there are at least 12 Yuan submarines of different variants (4 039As and 8+ 039Bs) in service with PLAN across 2 flotillas. They and the Type 039 Song submarines are the work horses of PLAN. After a rapid production run the last couple of years, the production has slowed in the past year. While this is the most capable of China's mass produced conventional submarine, it is not considered to be as classified as when it first came out. In the past year, Admiral Greenert, Chief of US naval operation, was allowed to go inside one of the Type 039B. While this generally reflect PLAN's effort to be show greater transparency with its USN counterpart, it also indicates 039B is not held with the same level of secrecy as Type 093 nuclear submarine. Since late 2013, a model of S-20 was displayed in various arms exhibitions. From one of the exhibitions, the S-20 is shown to have submerged displacement 2300 ton with maximum dive of 300 m. I was always under the impression that 039B was larger than this, so S-20 may turn out to be a smaller version of 039B.
In late 2013, It was reported that China had received order for 2 Ming class submarines (Type 035B) from Bangladesh. This was certainly surprising news since Chinese shipyard have not produced such submarines since early 2000s. Rather than selling 2 from its existing fleet, these were to be new builds. It's not clear which shipyard is building these submarines, since I have yet to see any pictures. While the type of submarine was surprising, the fact that China was selling to one of its traditional clients was not. Then in early this year, Pakistan announced that it will purchase 8 submarines from China along with 4 frigates. None of this was surprising, since reports of export of 6 to 8 Yuan submarines (S-20P for Pakistan?) had been rumoured for several years after Pakistan's U-214 deal failed due to funding issues. Since Pakistani Navy had always been purchasing advanced European submarines up to this point, it was significant that Pakistani Navy found Yuan submarine as suitable purchase. Even so, China's traditionally strong relationship with Pakistan was important in this deal.
While, the order from Thailand is not as large as Pakistan, it involved more competitors based on the various articles on this sale. With offers from Germany, South Korea, Russia, Sweden and France, S-20T won against some quality competition. None of this means Yuan submarine is the most advanced or the quietest conventional submarine out there. The article was very clear in that Yuan was picked because it the best value for money. In other articles, they also mentioned China's willing to transfer technology and provide training. I would think that other nations are willing to provide training and ToT also, so I think the bigger draw is China's cost advantage. The article also mentioned Chinese submarines can stay in the water longer and had superior weaponry and technology. That could mean Yuan submarine's AIP engine showed good performance in trials. The superior weaponry probably points to the torpedoes and submarine launched anti-ship cruise missiles that China has developed in the recent years. Traditionally, the Chinese submarines have been more noisy than western submarines. While this export variant of Yuan submarine is unlikely to the quietest in the competition, its cost advantage along with comparable performance in other areas won over Thailand. It's worth noting that China's 054A had lost out to South Korea in Thailand's frigate competition despite similar cost advantage. So this shows Thailand would not pick S-20 if it did not believe in its performance.
Conventional submarine is one of the most lucrative sector of defense industry. West European and Russian submarine makers had been winning most of the export competitions in the past, so it bodes well that S-20 could win one of such competitions on more than just cost advantage.