I sailed on the Horizon in ' 91 when it was new and again last year. The ship had been well maintained and I would not hesitate to cruise on it tomorrow. As a matter of fact, we had booked a 10 day cruise on it some time back but had to cancel.
I have also sailed on the Grandeur when it was new. The Grandeur is bigger, newer, has balconies, etc. but between the two, I like the Horizon. It doesn't have a lot of extras that the newer and larger ships have but it is a very fine ship.
I would be totaly comfortable sailing it again and again.But, that's just me.
I'm sure you will have a wonderful cruise on either.