Reason they look the same is color temperature difference. One is 7500k and the other 8500k. That 250w is as bright as a 750w standard life HPL. Put them side by side however, and the 250w arc will 'look' brighter do to color temperature. Drop some CTO in 250w arc to get its color temperature down to 3200k and they will look the same.
Likewise if you would have dropped some color, like a dark red for example, into the 575 and the 250 side by side, the 575 would have out shined it easily. See
here for where I explain quite a bit about color temperature and perception.
As for buying what you are looking for, of course. Always get what you need to do your job, never anything less. Otherwise you are wasting money. Some instances I need a VL fixture to do what I need to do, other times a Mac, still others a Studio Spot, and others a Design Spot. I simply said given a choice, I am taking a Design Spot series over a Mac series unless there is something I specifically need a Mac series for. For example, my client has custom gobos that only fit a Mac 2k.