Well, I haven't bought a Sony or iRex but got a REB 1200 instead recently (in addition to the existing 1100). For me to upgrade, the main selling points would be:
- A4 or half-A4 sized form factor (something similiar to iliad)
- touchscreen or some kind of input (a slave touchscreen LCD similiar to hanlin would also do)
- extensibility for developing applications similiar to the iliad
To be honest, the iliad meets most of those requirements, but it's too costly at $800. Something around $350 would be better.