We now know that the SSD on the nMP is user-upgradable, so your strategy for those might be different than for the laptops.
These decisions have always been economically driven, and the question is as old as computing itself. The answer keeps changing, because the cost of fast storage keeps dropping. I recall making the bold decisions to configure a PC with a 20mb HDD plus floppy, rather than dual floppies, and a few years later, to install more than the DOS-maximum addressable RAM (was that 512k, or 640k?) and to buy extended memory management software to handle the remainder of (gasp!) the 1mb of installed RAM.
For my money, your question (scratch disk on SSD or disk, inboard or outboard) should have been the one asked in these forums two or more years ago, but conventional wisdom is always behind the curve. To me, the notion that applications and OS require speed, but data does not benefit from fast storage is ridiculous, especially when you're talking about the large files we find in electronic media and databases.
Using SSD solely for OS and apps is the equivalent, in a kitchen, of storing all the pots and pans near at hand, but placing the refrigerator in another room. Now, in a commercial kitchen (and in a professional media studio) bulk storage demands are so high, and the number of cooks accessing the materials so large, that long-term storage has to be distant from the kitchen's core. The trick then is to fetch all the ingredients needed for the dish in one trip, and to organize them right at the cooking station along with the necessary tools ("mise en place"). You need a stove (CPU), a pot big enough to hold the dish as you prepare it (RAM), and a surrounding work area large enough to hold all the remaining tools and ingredients (SSD by today's standards). The distant, walk-in refrigerator and pantries should be visited as seldom as possible.
Should you add more RAM or a larger SSD? For the nMP 16GB of additional RAM currently costs $400, 256GB of additional SSD costs $300. SSD is 5-10 times faster than an HDD. The question for me isn't whether the scratch belongs on SSD, but simply whether that SSD should be outboard or inboard? Personally, I feel more comfy with it inboard, so I'd probably pay the "Apple Tax" (boy, do I hate that term, as if Apple was the only manufacturer who charges more for their memory components). I can't say an SSD on TB2 is necessarily going to cost most of us a whole lot in productivity compared to inboard. I leave that to Anand Lal Shimpi.
