That 600 fusion is a peppy piece of machinery- we own one. I'd go w/ 120. As someone noted, if you understud- you are more likely to rip 'em out. We have 120 in the 600, and I have 144 on my 800. I would not stud in the outside window, go down the center only. The outside ones definitely weaken the track and pull through much easier. Since studding center only- I've yet to have any issues. Tunnel protectors are a must to protect your machine. Also consider what size carbide you use. You should have a good balance- if you only run 4" carbides, but have 144 studs, your sled will push (not enough steering). If you run 10" carbides w/ 96 studs, your sled will slide (too much steering, not enough traction). If you run the standard 6" carbide, either should be fine, but I'd go w/ 120 if money isn't your concern.
As discussed in an earlier thread- I'm a firm believer in studs for the traction/safety. Not just acceleration, but more in the braking dept. Will it weaken your track? Maybe a bit. But I've had over 6K miles on a 144 studded 800 w/ absolutely no pull throughs or track issues. Will it eventually give out? Sure. But a new $300 track is worth the peace of mind I had when putting on those thousands of miles.