There's no doubt in my mind that the M2 Ultra's internal storage has a big advantage in speed over any kind of external storage available today. But then the question of just how much of an advantage that turns out to be in terms of resulting RAM (and perhaps CPU) economy for midistration applications remains frustratingly obscure.
Apple quote only RAM bandwidth – a staggeringly fast 800 GB/s in the M2 Studio Ultra – with not a word on internal storage speed. We do know from a few user lab tests of earlier M1 MBP and M1 Mac Studio that internal storage speed was phenomenal - in certain cases much faster than a Thunderbolt 4 port can handle. But also, it seems Apple have being playing tunes with their choices of storage devices in various M1 Mac models - some later devices being significantly slower than the fastest early devices. My guess is that the tradeoff Apple is quietly struggling to optimise has a lot to do with build-cost versus so-called "virtual memory" performance, primarily in video editing applications. But let's not forget that for realtime applications - including midistration - virtual memory is not an effective workaround for lack of RAM, even in the M2 Ultra Studio. At best, virtual memory can only relieve marginal shortages of RAM in offline (i.e. non-realtime) work.
(Virtual memory uses storage instead of RAM whenever the system runs out of available RAM, and can be, up to a point, pretty much transparent in video editing, but much less so in midistration applications. I recall when our large VAX 11/780 cluster regularly drove hundreds of our software engineers into sulky frustration because our corporation - believing DEC's hype about virtual memory - had been stingy about buying RAM and so the VAX cluster often went too deep into slooow virtual memory!)
I'm waiting to hear from anyone who's used a very large midistration template in a very large production on an M2 Ultra Studio - specifically about total RAM usage reported by MacOS. But since I've seen up to around 50 GB RAM usage in my 6 year old 64 GB system, and because the price difference between 64 GB and 128 GB RAM isn't too colossal (currently £800), I'll be going for the 128 GB option.
As for internal storage options, I'd be wanting 8 TB. That's because currently my libraries are pushing 5 TB of external SSD, and on top of that I use 2 TB total for dual-bootable MacOS & applications-dedicated R/W SSD storage. (For printing audio and for backing up, coupling externally to today's fast, LARGE-capacity spinning-platter hard drives - e.g. Lacie D2 - will do nicely.)
Above all, I want BIG and FAST realtime sample streaming without slaves! And I believe the time for that has arrived thanks to the M2 Ultra Studio.
So all told, I'd be looking at forking out an extra £3000 for my preferred RAM and storage options; total:- £7200. OUCH!