Hey Iceman,
First, a few things that will hopefully help clarify your questions on versions:
HSSE2 is a limited version of Halion Sonic that is included with Cubase 7.5. The Absolute + Collection is an additional product you can purchase and currently includes Halion Soinc 2, Padshop Pro (including Zero Gravity expansion pack), Retrologue, Dark Planet, Triebwerk, and Hypnotic Dance.
Halion 5 is our top of the line sampler, while Halion Sonic 2 is our muisc production workstation/synth. A license of Halion 5 can open Halion Sonic 2. H5 can open all of the same content from HS2.
The big difference between H5 and HS2 for most users lies in this question, “do you want a sampler?” By this we mean, do you want to be able to import your own sounds, manipulate them in H5, and play them back as a sampler would (pitch changes, layers of samples, etc)?
If your primary option would be to create a virtual instrument that plays samples you’ve created, I’d recommend H5. If your primary option would be to maximize the number of included sounds and instruments, I would recommend the VSTi Collection. I hope this helps!