SQL Server will typically optimize the
IN to the same as the multiple
OR statements. Take this for example:
where BusinessEntityID = 153
or BusinessEntityID = 25
or BusinessEntityID = 37;
where BusinessEntityID in (153, 25, 37);
Looking at the execution plan I can see that SQL Server treated them as like statements and used the same execution for them both:
Ultimately, though, you need to do the same exercise in your environment with your queries and your data (if you have differing plans for these different variations, feel free to post the post execution plans in your question and I can analyze). And of course like all things relational data, you need to ensure that you have the proper querying and indexing in place in order to optimize the data retrieval.
The other side of that is readability/maintainability. Consistency is king here for development teams, and if this is a solo code base then I'd tack on personal preference to that.
IN is a lot less verbose with characters, and the typical choice.
Just a note, though, if these conditional checks get out of control (hundreds of hundreds of elements in the
IN clause, I've seen it before way too often) you might need to step back at what you are really doing and refactor your logic.
Your actual error is because of a different cause though...
It is worth noting though that your actual error message is a connection timeout, not a command timeout. It looks as though you've exhausted all pooled connections in your connection pool for the application. Take a look at this blog post I wrote on connection pooling. That should explain why you're getting that error and how to fix. It is very possible that other sessions are running too long, and not closing or disposing of the connection to return it back to the pool. You would most definitely need to take a spearheaded approach in troubleshooting your connection timeout issues due to exhausted connection pooling.