Z80, why change? You know microchip very well, so you have a good point to start from. Microchip is good expecially for non-complex applications.
BTW, I prefer holtek mcu, which are less expensive than microchip,and more and more companies in Italy ask for holtek MCU for this reason. Also, IDE environment/compiler is free, the ICE has a very very low cost (less than 300$) and parallel programming interface provide a very fast programming time (ok for mass production).
Holtek develop only simple MCU, with no more than 8kword (RISC architecture) of ROM, and is not Ok for firmware that need to store lot of strings in ROM (LCD or SERIAL alphanumeric communication?), so for some application I use Nex 78k family (powerful CISC core with some 16bit registers and powerful peripherals). This is a great micro, but due to the software flash programming algorithm, need lot of time to be erase/programmed: this is the only thing I detest of this micro.
Also, tools like compiler and programmer has a high cost... but if you need, you can find a Nec 78k MCU programmer for free from my site
https://www.creasol.it/equipment : the software is available ini my site, works well in Linux, not very well in Windows using cygwin posix libraries. I hope that someone that has familiar with windows programming should port the software to windows!
I've used for a month Motorola HCS12 MCU... it has an architecture like Nec 78k, but has three great advantages over Nec:
1. It is very fast to be programmed
2. the flash should be programmed by the firmware itself
3. a very very very great compiler, Metrowerks Codewarrior, is available for this MCU (free of charge till 12Kbyte of code): it is the best compiler I have seend, with strong optimization...
Bye. Paolo