9910 and 8910 are the same except for RF.

8810 and 8811 are the same.

8811 does not have LCD.
8811 has a better key layout that 8910.
8811 has an extender (software) available to allow more customization.
They have the same amount of each type of memory. With JP1 you should be able to fit almost anything reasonable you want in that memory. With the extender, you can fit even more into the same memory. If you want to really go overboard, you can replace the 8811 eeprom with a larger capacity one (if you can solder that well). You can't use the 8910 with a larger capacity eeprom.