I have recently started to use my Macbook 4,1 again, even after getting my Surface Pro 2. There are a variety of reasons why, but that is for another time. I decided to try Debian and Mountain Lion, and I have found out which OS is for me.
Debian
Debian is a great operating system. It is free and open source, and it is very stable and secure. Getting it to work on my Macbook was a bit tricky, but I managed to do it a while ago. It was great for the time I was using my Macbook as my main laptop, except for a few problems. The trackpad was very broken using Debian, as it would spazz out and would often click things I didn't want to click. It also deleted my text while typing very frequently, which would leave me frustrated as I had to retype whatever I was typing. Overall, Debian was good for getting newer software to run on my Macbook, but it wasn't the best experience.
Mountain Lion
OS X Mountain Lion has been unsupported by Apple for a long time now. It was released in 2012 and stopped receiving updates in 2015. I managed to get it to run using NexPostFacto, a patcher that allows older Macs to run Mountain Lion with graphics acceleration. I use my Macbook mostly for online tasks nowadays. However I did do a bit of GIMP work on it too. Although iMessage and FaceTime don't work, I really like Mountain Lion, since it doesn't have the same issues I have on Debian. The keyboard doesn't randomly try and delete text and the trackpad works really nicely. I also prefer the older look of Mountain Lion than the newer flat design of newer operating systems. Overall, Mountain Lion has been great and reliable for me on my Macbook.
Conclusion
If you are looking to run modern applications on your older Macbook, Debian is the right operating system for you. If you are looking for stability and reliability with older versions of applications, Mountain Lion is good for you. I have really taken an enjoyment to Mountain Lion compared to Debian on my Macbook. I hope to be able to get more life out of it, even though I have moved on from the device.