I have flown an Astro Hog with an OS 70 Surpass and it had good performance. Nice big loops, would not go vertical, lots of 2/3 throttle flying.
I have also flown an Astro Hog with an OS 91 Surpass and it has excellent performance. Loops as big as you want, almost unlimited vertical, lots of 1/2 throttle flying.

Both engines you have should do a very good job, if you want the vertical capability go with the 75, but if the vertical is not required the 55 will pull it through big loops.