Both the Inspiration and Sensation are sister ships so, except for the superficial differences in internal decorating, they are basically the same inside. The Sensation is three years older than the Inspiration, not that it makes much difference.
I was on the Inspiration in 2005. I have never sailed on the Sensation, so I can't tell you much about it (except that it is probably very similar to the Inspiration).
I had a great time on the Inspiration. The food was very good. Not that it would hold up to 5-star cuisine, but it was more than good enough for me.
The entertainment on the Inspiration was very good (in my Opinion, which may differ from the opinion of other(s)here). I was amazed by a Russian dance couple "Demtri and Svetlana" who might even impress their fellow country-man Barishnikov. The cruise director (a big burly man) impressed me with his role as Belushi's charactor in their rendition of the Blues Brothers.
What I liked best about the Inspiration was the ports of call (destinations). I liked Grand Cayman (although we were there for just a short time). I really liked Cozumel, Mexico. The shopping was great. I always felt safe. The street vendors were never pushy. I heard the diving and beaches are really great (but I didn't have time to see them).
The Sensation goes to the Freeport and Nassau, both in the Bahamas. I never been to Freeport, but I have been to Nassau twice on other cruises. Unfortunately, the street vendors were pushy. The shopping in Nassau was OK (Cozumel was better). I guess I was safe in Nassau (but sometimes I did not feel that way). Although, I did have a good time at the Atlantis Resort across the channel of water from Nassau. The aquarium in the Atlantis Resort was amazing.
So, if I were to choose either the Sensation or Inspiration, I would (IMHO) choose the Inspiration. Primarily, since I really enjoyed Cozumel.
I hope this was some help. Have a good time on your cruise.