3.1 is much simpler than 2.5 or 2.6, but currently suffers a severe dearth of third-party add-ons, environments supporting it (big apps using it for scripting, etc) and tools such as IDEs. So, much depends on what you want to learn Python for -- if just for personal edification, 3.1 is ideal; if it's to actually build or control applications, websites, etc, then 2.5 or 2.6 will serve you better at the present time (3.* will no doubt reach and surpass 2.* in the future, but, the future is not here yet;-).
