Since this only happens during periods of idle time it doesn't matter that it takes a little longer to compress the files. The files will still decompress very quickly. You can have SpaceSaver automatically compress files after a period of inactivity which is based on the files modification date. For instance you can have SpaceSaver compress all files that haven't been modified in the past month. The only catch here is that some files never get modified but they still get used.
Applications are a good example.
So if you like the idea of a saving disk space without any hassle then SpaceSaver or a product like DoubleSpace is worth a look. If you like the level of control that SpaceSaver gives that DoubleSpace doesn't then check it out if you prefer the DoubleSpace way then it may be worth the extra money but remember SpaceSaver is free with the purchase of StuffIt Deluxe. I still like having that extra bit of control that SpaceSaver gives me that allows me to tell it to leave the critical stuff like the operating system and the programs I use most alone. I just like trying to wring that last drop of performance out of my machine. I think the choice of the two also relies significantly on your machines configuration.