M12 vs M18 Brad nailer?

I’m a hobbiest at best. But have a project coming up. I’m building my wife a library in her office and doing built in bookshelves. There’s several applications throughout the process where a Brad nailer would be beneficial. I’m pretty sure I could get away with just using 1 1/2” Brad nails for the majority of tasks in that project as well as throughout my house. Just stuck on which would be a better option. I have plenty of each type of battery so I’m just stuck in the middle. Plus if I’m using it for a long stretch of time, the m12 would be alot lighter. But the m18 would be more powerful and the battery would last longer.

Anyone got advice?