I use an older one because I don't want to re-learn the new one, but if you can use the new one, then do it.
I'd strongly advise against that. Newer versions of the G3D exporter (which also need newer versions of Blender), for example, allow features like proper toggling of double sided textures (older versions did it wrong), toggling of whether the mesh is selectable, etc. As well, Blender itself keeps getting new features, and eventually documentation for older versions of Blender will be completely replaced. Since you're just starting modelling, I strongly recommend using the most current version of Blender. At the moment, that's 2.63a, but I'd personally advise updating to new versions when they become available, as they fix bugs and are generally backwards compatible. It's true that the jump from 2.4x to 2.5x (and newer) changed the GUI drastically, but after using it for a period of time, I've come to the conclusion it's a beneficial change (there'll always be some people who don't want to embrace change, though, even though 2.4x's GUI was rather confusing and nearly forced the usage of hotkeys).
At any rate, the differences between the versions you have (2.57 to 2.63a) are minimal: mostly bug fixes and new features, nothing near the scale of the GUI change that the 2.5x betas introduced. There's pretty much no reason to use the older versions there, and the latest G3D exporter script ONLY works with 2.63a and higher.
Although really, we can view most programs in the same way: there's no reason not to update Firefox, for example. Sure the new (well, not really that new) versions changed the GUI drastically (I believe it was Firefox 4 that did that), but newer versions are much more secure and streamlined. GUI changes can take some getting used to, and can sometimes bring up questions of "if it ain't broke, why fix it?", but overall, I'm in the opinion to just suck it up and update unless there's some strong reason not to (eg, XP to Vista was a questionable upgrade, but there's absolutely no reason to continue using XP over Windows 7 except to be stubborn).
