Not wishing to be argumentative, but why buy both? I have used the 2 reel tanks for years with the vast majority of films being developed on their own. I feel it's handy to have the option to do 2 films at a time and I also do 120, although I didn't when I bought my first tank.

It is handy to have multiple tanks, more so multiple spirals (if plastic) as their b******ds to load when damp, let alone when wet. It all depends on how much film you expect to process.

To answer your specific questions in one, for a single film, both the single spiral tank and double spiral tank would use the same quantity of chemicals.