Actually learn both so you can make the right long term decision for yourself.
For now just pick which you will learn first.
EntLib is more of a just foundation library (like basic building blocks) while Spring.NET is both foundation stuff as well as more specific pieces (i.e. ASP.NET).
EntLib is being actively developed and maintained by MS, while Spring.NET is a community effort which currently is a bit stale and slowly becoming rusty (i.e. lack of container configuration via code - it is being developed, but without a real release)
Both have good documentation (which is not so common for OSS libraries).
Spring.NET has excellent docs, EntLib has pretty good but not always the best (sort of MSDN style where often you will find code samples that are too simplistic to present the idea).
I think EntLib might be a bit easier to start with, but as I said learn one and then try the other to make a smart decision cause you will definitely have personal preferences and like one more than the other.
UPDATE:
Spring.NET is being developed more actively now, realases are more frequent, Spring.NET CodeConfig in now official and past 1.0 also new projects appeared like Spring.NET Social and Spring.NET REST Client.
There's .NET 4.0 build, it supports MVC3 and NHibernate 3.2 so basically it looks like the project is not dying at all - quite the opposite actually :)