The J139 is a 9504 in SIP form. I haven't seen any missing features from a functionality standpoint, though the background and screensaver images can't be changed, unlike the J179. I don't know prices but personally if price is an issue for the 179 I'd drop to the 169 instead of the 139 as I prefer the 8 button model.
The J169 and 179 are 9608 replacements. The J169 is gray scale and the J179 color plus the 179 has the optional WiFi/Bluetooth module.
I concur with TheSmash that the J129 is best as a lobby phone or a similar use though I do think it is actually a pretty nice unit for that use. With the optional WiFi you can place it anywhere there is power which could save you some difficult cable runs.
- Qz