Strongly biasing your decision to use ARM rather than MIPS because ARM has a significantly larger number of vendors doesn't hold a lot of water.
I was mainly referring to the various vendor-proprietary cores you run into now and again.
If you think there's much portability between ARMs from different vendors you're in for a rude awakening, the way vendors differentiate themselves is by what value they add on to the part, and that should usually be the primary decision maker, far more so than whether it's MIPS or ARM.
Everything except the peripherals is transferable knowledge. That includes the architecture, the core peripherals, the compiler, the debugger... That is not insignificant.
I agree, it's not insignificant, but it's really pretty minor compared to whether a part is fit for purpose.
We mustn't forget that there are also quite key differences within ARM implementations even within the same vendors, such as cache, varying wait states for different memory, differing memory segmentations, differing and sometimes absurdly complex clock schemes, all of which I'd say you'll encounter before you care about whether it's ARM or MIPS. Even saying "ARM" is a very general term, when we are to consider M0/M3/M4/M7 as well as the Cortex A and Cortex R ARMs.
As an example, say I wanted an on-chip high speed (>10MSa/s) ADC the obvious first port of call might be NXP's LPC4370 ARM Cortex M4F. However, say I want multi channel simultaneous sampling, pretty much the only option is the PIC32MZ EF.
One other facet also sometimes overlooked in device choice is leveraging the existing skill set of your development personnel, and whether there is a reasonable pool of skilled individuals out there to deal with normal churn rate. The cost of re-skilling and purchasing new tools is commonly not to be sniffed at. That is also a key reason why I find some vendors' recent examples of pushing proprietary frameworks so baffling, they are placing a extra unnecessary barrier in the way of selling their devices.
So that's why, in my view, the choice of core should be a relatively minor consideration among many others when choosing a device.