Deploying Assembly in GAC vs BIN

If you are frequently updating the Assembly, then you always better to deploy it in BIN.  Since, Assembly will be reloaded automatically just after the updating.    But when you update the Assembly in GAC, you have to restart the IIS (IISRESET) to grab the new version. The reason is GAC keeps the assembly in Cache.

When you deploy your Assembly on GAC then you can access the Assembly from any SharePoint web application. But when you deploy Assembly in web application’s BIN folder, then it can only access from the given web application. Anyway, if you have all-purpose web part, you better to deploy it in GAC and avoid the multiple Assembly deployments in BIN. 

If you have multiple versions of same Assembly, then you have to deploy it in GAC. Coz, GAC manages the multiple version of given Assembly, but BIN doesn’t