I went ahead and spent the evening putting the latest versions through their paces.
IMO:
AstonShell is significantly better than the current Talisman. 
Things going against Talisman:
- Felt sluggish
- Default themes buggy
- Style over substance (i.e. everything seems to be made to look pretty in a screenshot rather than actually work well)
- Poor system tray support
- Shell is less featured in core areas than Explorer (no drag and drop off menus, no taskbar grouping, etc.)
- Seems complicated to customize
Aston, by contrast had these points:
+ Felt snappy and fast
+ Seemed more polished
+ Seemed less system specific. The themes worked better on my system (taskbars didn't get messed up simply because I was running a lot of tasks)
+ Good system tray support
+ Theme chooser more straight forward
+ Easier to undertand
Going against Aston was:
- Felt less powerful than Explorer (no taskbar grouping and other things)
- Lost my desktop icons you don't really have a "Desktop" anymore as far as I could tell
- Goofy default directory c:\aston
- Includes a quasi-warez theme in there (Aqua theme compelte with Apple logo)
- Themes feel pretty static (like Talisman).  No drag and drop anywhere.
Overall, Aston felt closest to being a professional level product.  I don't think I'd use either one on a regular basis (I really don't like them taking away my desktop icons). The main reason though is that I'm used to Explorer so I'm more interested in extending that rather than replacing it.
But I can definitely see why Aston is preferred by many.