Well, it is hard to say, nanking is 100 km inland, it means any carrier aircraft inbound will be easier to detected, loitering time of combat aircraft is less, navel asset can only assault nanking from one side instead of 2 sides for shanghai (as the japanese have exploited with their marines).
The approaches to nanking also is much wider than shanghai, partisan will have more opportunity to attack, Japan will have to move more distance in hostile territory which means more fuel used in locomotion which might translate to weaker supply to front line japanese troops.
would all these make a difference? i do not know as these are all subjective. who knows? would the nanking city wall help the defender? atleast you force the japanese to bring their heavy artillery.
Why would the national morale fall more with shanghai than nanking? historically, it had always been the capital which held more sway than a major commerce port city. I would reason the fall of nanking is a larger morale breaker than shanghai - most chinese dynasties fell with the capitol city, same is true for most European kingdoms and Arabic states.
Which looks worse internationally? a country which could not defend her own capitol or a country which could not defend her largest port city?
a fighting retreat is not easy to execute, Mao did an excellent example during the long march. The Japanese are not that motorized as well and that the Chinese are mainly light infantry again with high mobility. Sure the Japanese have shown they were able to bypass strong points in the pacific campaign, but in doing so they have also exposed their flanks and over extend their supply lines. So it is definitely not an end it all maneuver.
I don't think man power is an issue for china, and fighting retreats do not require huge amounts of men power. Rommel had many fighting retreats with inferior number of tanks and infantry while under enemy air superiority. If the cream of the German trained national army was in shanghai, can we expect a fraction of the German infantry ability from them?
On hindsight, I realised that fighting in Shanghai was more advantageous for Nationalist China as opposed to fighting in Nanjing. And here are the reasons why:
1. Shanghai was the equivalent of a beach-head for the Imperial Japanese military. Hence fighting in Shanghai meant that the Japanese were being engaged at the beachhead with hardly any reserve area away from the fighting from which they can rotate troops from.
2. Fighting in an urban city provided a lot of cover from air attacks. This negated to some extent the Japanese superiority in the air.
3. The Japanese advantage in armour is also somewhat negated in urban fighting.
4. The Japanese advantage in artillery is also somewhat negated in a city since the buildings provide protection.
5. In urban fighting, it comes down to familiarity with the area and close range fighting. And Nationalist China is at parity or a slight advantage in these areas.
In contrast, a fighting retreat to Nanjing means exposing the better trained troops to ceaseless air attacks by land-based bombers, armour and artillery. It also allows the Japanese military to field their heavier equipment (i.e., armour and artillery) in open terrain, which is what they have been doing in Manchuria for years. It also leaves Nanjing open to land-based artillery attacks as the Japanese forces draw nearer. I'd say that's a rather bleak situation even if the fighting retreat was successfully conducted.
As for the issue of a lengthening supply chain for the Japanese, the distance between Shanghai and Nanjing isn't too much of an issue as the Yangtze river provided a waterway with which replenishment can be carried out with minimal interference.
@delft
The Japanese approached Nanjing from the south, southeast and east of the city, which is essentially land route. Nanjing's west and north lies the Yangtze river, a natural defensive barrier that became a trap for the people in the besieged city.
