Just my personal opinions here (I have CS4 and Giles)
CS4
Firstly, I would wait until CS5 is released before considering shelling out on a soon to be old version. CS4, I have found, is limited by two main things, which make it pretty redundant: Firstly, you cannot import 3d models, created in a 3d modeller, into CS4 properly. (You can import them as a mesh, but they loose all their uv data - so textures get lost and you can't reapply textures to individual faces.) That means that any objects you want to put into your level must be created within CS4 - and CS4 is no modeller. You can create basic objects, but nothing as intricate as in a dedicated modeller. Secondly, (in my experience) the light mapper works fine for objects with flat surfaces - but really badly on objects with curved surfaces.
Giles
In Giles - you CAN import models from a modeller into Giles with all their uv data intact. (You can create primitive models within Giles, but even more limited than CS4). It DOES lightmap curved surfaces really well. The only problem, is the export to DBPro. There are two ways (as in CS4) of doing this: Export as two models - a .x object for the model, and a second .x object for the lightmapped object, the latter being ghosted over the top of the former within DBPro - costly on performance though having two models and twice the poly's. Or, you can export as DBO - one single, lightmapped object. Much better for performance, but at the moment, the dbo exporter doesn't work reliably - bits of the scene go missing, or have positions changed. I think the author is working on it, though.
So, personally, I would either wait to see what CS5 has to offer, or get Giles once the dbo exporter is working properly.