An upside to Leopard is it's customization abilities. On my G4, Leopard runs at a good speed, good enough to be able to run 4-5 applications before slowing down considerably. It can do anything most modern OSs can do, while still being able to run on a Processor under 867 MHz. Mac OS X v.10.5.8 Leopard: Leopard is a very modern OS, even today.
Tiger can still do the basics, word processing, web browsing, and so on, so I really don't think Tiger is all that bad. A great feature that Tiger has the Leopard does not, is Classic Support, which I find very helpful at times. It also looks really nice, for an older UI. Along the lines that Tiger is faster, I have to argue that the Finder is much more responsive and fast than Leopard's Finder. With RAM upgrades and and a newer hard drive, Tiger is pretty darn snappy and responsive. A lot of people say that you're crippling a PPC processor by running Tiger on it, and if you have a G5, that may be true, but most G4s under 1.2 GHz perform much better under Tiger. Tiger is also faster on most processors than Leopard, although I know people will disagree with me. Tiger would fully load with the screen shut before the laptop went to sleep, whereas Leopard always sleeps before it is fully loaded. I've also experienced this because, on my 1.33 GHz G4, I had it set to automatic boot up. Many people debate over it, but it honestly really does. Mac OS X v.10.4.11 Tiger: on G4 processors, Tiger definitely boots faster.