You can do either. Mix it up. Both are good.
10 x 100 ON 2:00, so first one would be, say, 1:50 + 10sec rest, then try 1:45 (then you get 15sec rest). The nice thing about doing it this way is that you get more rest the faster you go.
You can also do an interval with a set amount of rest, regardless of how fast you go, such as (my favorite) 3 x 800 with 2:00 rest.
Masters Teams tend to do intervals "ON" rather than "WITH" because it keeps everyone swimming together. But they both have value. I know that Jensen (and this in unbelievable) once did 20 x 1500m. He had to finish each one under 18 minutes (yes, that's right) and then got 2:00 rest, regardless of how much under 18 minutes he went. He finished 15 of them, but then couldn't make it under 18 minutes. So ridiculous. That workout would have taken almost 7 hours to complete... So there are examples of both working well for people. Do both, mix it up, keep your thoughts fresh.
The key is just to swim faster as the set progresses... You can also mix in a ladder. Like if you are doing 10. Do 1-5 descending, and then slow down and do 6-10 descending (although do #6 faster than you did #1).
"Non est ad astra mollis e terris via." - Seneca | rappstar.com | FB - Rappstar Racing | IG - @jordanrapp