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Q: A Few API Related Doubts My first is related to Enterprise WSDL vs Partner WSDL. I know the basic difference between two: tightly coupled vs loosely coupled, one is tied to specific organization while other is used to build client applications, etc. My question is why use Enterprise WSDL, even to integrate with my org? What is its advantage over Partner WSDL? I can use Partner WSDL even to integrate with a specific org.  I used it 'some time back' to create  records in my org and it worked pretty well. 
Another thing : 
Salesforce suggests to use Bulk API if records are to be created or updated in large numbers. But today I tried to upload around 1500 new records in batch of 200 records using the following, which all worked: 


*

*Enterprise WSDL

*Partner WSDL

*REST (@RestResource annotated Apex Class)


So where is the problem? One potential problem I see is the daily limit of APIs call into an org, but even if post say 100K records daily, number of calls would be 100,000/200  =  500 calls, which one can have even in a developer org. 
What other issues might I come across if I use SOAP API or REST API for bulk posting of records? Is there an approximate record count where one should start thinking to use BULK API over SOAP/REST?

A: The bulk API is ideally for much larger than 1500 records. 
Like you state, in cases where there are 100k of records a typical use of the SOAP API would quickly begin to eat away at the daily API limit for an org. And this is really what you should be aiming for. 
When I used to teach this in DEV502, we would state (somewhat arbitrarily) for over 50k records you should definitely use the Bulk API. Would the SOAP API work? Sure. Would it be less efficient? Definitely. Would it use more requests? Yes. 
As for Enterprise versus Partner, I get the sense that when push comes to shove most integrations use the Partner WSDL. Again, this is a question of trade-off. 
In theory, you could probably get your integration up and running faster with the Enterprise WSDL and its explicit custom object/field types. But you pay the price of lock-in to that WSDL. 
Alternatively, the Partner WSDL might take a bit longer to implement initially, but you have greater flexibility. 
Most integrations decide to pay the price of longer initial integration for greater flexibility in the future. 

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