Interesting catch. It does appear to be a contradiction. If I recall correctly, the Insight books and the Revelation book were released around the same time (1988/89). Of course, the bulk of the material in the Insight books is directly lifted from the old Aid book (1971), so perhaps it's some "old light" that wasn't corrected during the editing process.
In general, though, I find Revelation 14:1-4 to be a fun topic of discussion with JWs. In short, the WTS teaches that the 144,000 mentioned in verse 1 is a literal number, but in verse 3, it's symbolic (it represents the remnant on earth during the "last days"). Most JWs don't know this. If you're feeling frisky, try the following:
Me: So when the Bible mentions 144,000, that number is literal, correct?
JW: Yes.
Me: (turn to Rev. 14) So when verse 1 refers to 144,000 that is literal?
JW: Yes.
Me: and they're in heaven, right? (verse 1 says they're with the lamb on Mt. Zion)
JW: Yes, they are.
Me: Ok. How about verse 3, what is going on there?
JW: The verse says they're singing a new song before the throne and the 24 elders.
Me: Who are the 24 elders?
JW: They represent the 144,000 serving as kings and priests.
Me: So they're singing before themselves? How does that work?
JW: umm, the Revelation book says that the remnant of the 144,000 is singing in front of the already-resurrected 144,000.
Me: So, let me get this straight. In verse 1, the 144,000 is a literal number of resurrected people in heaven, but then in verse 3, the 144,000 means a much smaller number of people who are still on earth?
JW: Yes, that's what the Revelation book says.
Me: So, in other words, you believe that 144,000 in the Bible is a literal number sometimes but not always?
JW: This is pretty deep, let me do some research and get back with you...