which to buy (cant find in faqs or lists)?

[malefactor]

I'm looking to get into this, start tweaking, and maybe even sling some code for this.  So here's the magic question:

What is the product differentiation?  Ultimately I want to use this in my car.  So which one of these has:

 - hard drive, preferrably replaceable
 - long battery life, preferrably replaceable li-ion
 - most "new feature" potential (untapped, or perhaps existing)

Thanks in advance.  I've dredged through FAQs, forums, and lists to no avail.  Perhaps it would be useful to add this to the FAQ with a summary chart:  weight, size, capacity, distinguishing features for each model?

[LinusN]

Try the documentation section on http://www.rockbox.org, especially the Feature Comparision Chart:

http://www.rockbox.org/docs/features.html