One advantage of choosing a '51 based chip is that the tools are made by a toolmaker, e.g. Keil and the chips are made by a chipmaker e.g. Philips. That way you get both parts from someone that has expertise in their field. Most (all I know) toolsets provided by a chipmaker for their unique architecture are severely lacking.
Another advantage of choosing a '51 based chip is that if the chip can not handle your load, there is always another 'similar' that can (unless, of course, you start with a 100MHz one clocker from Cygnal). An example of this is that my P89C668 based design ran out of steam as features were added, replacing it with a F126 was a nominal process, not a total redesign.
So, In my opinion, there is only two choices '51 based or ARM based.

Erik

Neil, have you looked a Cygnal, they might just have what you need.