Beside all the known features already available in CCleaner, SlimClear has some optimization modules bundled, with which means you have the possibility, as they say, to increase battery life, to optimize system processes, defrag the HDD, etc. These could come in handy, but CCleaner keeps it simple by removing only the temporary files, clean registry items, wipe the disk, monitor software installation, etc.
It is up to you to choose what version is the best for your computer. I am using CCLeaner for a very long time and have no plan to replace it.
Additionally, you can increase CCLeaner's functionality using CCEnhancer. CCEnhancer adds search strings to Applications tab. Basically, it allows you to perform in-depth cleaning for apps and features available in Windows.