SQL - IN vs. NOT IN

Use "IN" as it will most likely make the DBMS use an index on the corresponding column.

"NOT IN" could in theory also be translated into an index usage, but in a more complicated way which DBMS might not "spend overhead time" using.

I have seen Oracle have trouble optimizing some queries with NOT IN if columns are nullable. If you can write your query either way, IN is preferred as far as I'm concerned.

When it comes to performance you should always profile your code (i.e. run your queries few thousand times and measure each loops performance using some kind of stopwatch. Sample).

But here I highly recommend using the first query for better future maintaining. The logic is that you need all records but 9 and 10. If you add value 11 to your table and use second query, logic of your application will be broken that will lead to bug, of course.

Edit: I remember this was tagged as php that's why I provided sample in php, but I might be mistaken. I guess it won't be hard to rewrite that sample in the language you're using.