Requesting early publication is not very common, I'd say it is a strategical move above anything.
There are two advantages that I can think of. One is the evident one, making your patent application to be comprised in the prior art with an earlier date. The other advantage, if it can be seen as such, is provisional protection if the law in the country that you are to enforce it has such provision. This is enshrined in Art. 29.1 PCT. I'd say there is no difference between cases a) and b) of your question because none of these advantages changes depending on the date of publication of the PCT, the date of publication of the ISR, and the outcome of the ISR.
There are several disadvantages. The evident disadvantage is that your own prior art can destroy a subsequent application of yours. Should you file a second application claiming priority from the published one and include new matter, for example to address patentability issues or to disclose new embodiments, the new matter will have to be both novel and inventive. If you did not file a patent application in a non-PCT member state (e.g. Argentina) but suddenly wanted after the priority year, the early publication will anticipate your claims if you file a patent application between months 12 and 18 and the PCT is already published.
Another important disadvantage is that you cannot withdraw your application to avoid its publication if you were to change your mind. Say you are not interested in the invention anymore for whatever reason and you do not want to make it public. The 18-month window (minus 15 days) give you this possibility. From my experience there are many applications that are withdrawn and shelved. For this, the ISR is sometimes a game changer: relevant prior art will affect the maximum scope of protection you can get, should the scope not be of your interest then you might make the decision of abandoning your PCT.
Less important disadvantages I can think of are the following. Your early-published PCT application can be monitored earlier on by third parties, more observations can be filed against it. And requesting early publication of a PCT is subject to the payment of an official fee.