The is a subtly noticeable difference between 10.4.9 and 10.5.5.

Emiliano is correct. The updates in 10.5.3 helped things. There was another update in 10.5.5 which helped with a few hiccups.

The bottom line is that if you have Tiger, use tiger. If you have Leopard don't worry about going backward to Tiger. Leopard is fine but make sure you are higher than 10.5.3. So far I have seen no problem with running all the updates to 10.5.5.

There are quite a few other threads where Richard has indicated the performance characteristics on different versions of OS X. I suggest you read them if you have any specific questions.

Right now, the simplest focus will be to get an SSD disk like an Mtron. The disk access time is where you're going to notice performance.