I will use C and also will try to re-use a lot of C-code between architectures.
I do not know what you are intending to make, but in the microcontroller world the ability to reuse code is severely limited since so many things are architecture dependent. And the 'universality' of C can not automatically adapt to architecture changes. There are many that think the microcontrollers are 'small x86s' if that is your belief you are sadly mistaken. You will find that several applications will have a mix of C and assembler, simply because C is not efficient enough to handle that extremely time critical things microcontrollers are intended for.

I 'smell' that maybe you are going for the " '51 PC" if so, cut your losses and stop.

If, however, you are indeed interested in the applications for which microcontrollers are intended, I do not see why you will need bankswitching except in the most extreme cases, so start off with 64/64, it will handle 99% of anything the '51 is the right choice for.

If, however you decide to push forth,
So to start from the other end, which compiler is the cheapest (with good reputation)
that do support bankswitching?
Raisonance - which I am not using, but would if I started a new shop - has two advantages over Keil
1) they are cheaper
2) they do not charge extra for Mx and 669 support

There are Raisonance users in this forum and no 'whining' has been posted.

Again re bankswithing: Abandon it, it will get you in timing trouble - and if timing is not a problem why are you using a microcontroller - the Philips Mx and 669s plus offerings from other manufacturters are much better solutions for large memory '51 applications.

Enough

Erik