Basically, as has been said here, inlines are more delicate and heavier than radials. The flip side is that inlines are better aerodynamically.
The Japanese had three aircraft with inlines, the Tony, Judy and Seiran. Due to a lack of experience with inlines and lack of materials for making seals they did not do well operationally, though it is worth noting that both Tonys and Judys were outfitted with radials later on. A slightly slower plane in the air is better than a fast one in the shop!
In Europe, the very strong trend was to inlines. Longer experience with them and less over water flying I guess. The only radials were the Fw190 and some Russian aircraft such as Lavochkins.
The US Army Air Corps seems to have gone with almost exclusively inlines for fighters and radials for bombers. I am going to guess that lightness for bombers to allow more load and to give fighters more speed. Funnily enough, they did not want superchargers on their inlines which is why the P40 and P39 were sluggish at high altitudes.
The reasons for any particular engine being used is partly tradition in the service in question, availability and in some cases, performance!
Joking aside, you can make all the general rules you like and there are exceptions to all. The 190, Sturmovik, P47, La5/7, etc are all exceptions to conventional wisdom regarding engine choice. Yet all were extremely successful.
None of this gives you a definitive answer but I am sure you are getting that there are no hard and fast rules and that such decisions were made on a case by case basis, often not for purely aerodynamic reasons.
I hope that helps,
This message has been edited by ccowx from IP address 220.127.116.11 on Mar 11, 2012 12:02 PM
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