Karen,
My opinion really is very biased, but I'll give it to you anyway

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Some history: I have seen all of the Genesis shows (with and without Peter Gabriel), so when I first went to see the Musical Box, I was excited to be re-living my early teens and experiences with my favorite band ever (Genesis) and yet I was a bit skeptical.
I first saw the Selling England By The Pound tour and was absolutely blown away by how great the show was. Every single detail was just perfect. I was soooooooo very impressed by it. From that point forward, I have made attempts to see TMB whenever they were/are in the Philly/NJ area. I've seen several shows now, several times.
The Lamb show (which is what I think you'll be seeing) was also very impressive and also brings back some great memories for me.
I have seen the Trick of the Tail show also and although it's very good, I don't feel the need to see it over and over again. But still, a very good performance.
The only advice I can offer you is this: you must look at the show and keep in mind that it's a representation of what they did more @ 30 years ago -- they use everything just as it was, so no modern-day technology -- however, I think that's what makes it so special and makes you realize just how advanced Genesis was back in the day -- progressive, I guess you could call it

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I think you'll enjoy the show, especially if you've listened to the Lamb and have liked it.
Let us know what you think afterwards and have a great time.