Well, after thinking about it, I decided to keep on with Microchip, probably going for 24HJ pics,
which deliver a quite high performance of up to 40 mips, whereas many arm7 mcu will move around 50 mips.
Also, there's quite a lot of C source code available. Not sure if more or less than for pic.
The main problem I find is time. My time. Setting up all the open source software for the arm, and
some hours struggling to make it work fine (a few hours, or not so few, depending on many factors),
but basically not having a proper IDE and development software as I have for pic, is an important fact.
Many things to do, and I'd like to focus on the project.
Arm7 seems great, if you can afford a development kit, and make sure you'll have development software,
or otherwise you've got plenty of time or want to take the quest of sorting yourself out.
I didn't find a single pic with an external memory interface, neither in 18F series. So I guess I
should write a bootloader to load code from an SD card, and then reflash the internal instruction
memory.