The lack of syntax highlighting makes it a lot easier to make a mistake in Sugarcube. Harlowe makes it really easy, say if you forget to close a macro to notice it before you've gotten too far. I've even had to add a custom story javascript to do some things that are already built into harlowe. For example, customizing the menu bar in Sugarcube takes a couple of lines in there. Also, I always store my variable as all lowercase, and then if needed uppercase the first such as a 'he/she' at the start of a sentence. In harlowe you just use the built-in '(upperfirst: $variable)' macro in-line and it's easy. In Sugarcube, I had to add a custom story javascript to do that, and then it's referenced as a function. Once it's implemented it's easy to use, but took a good bit of playing around to get it working... but now I have a good template I think.
I am building mine in Sugarcube now, but am also working on it in Harlowe at the same time. Been a good experience to figure out the pros and cons of each. Maybe I'm just sadistic.
If you get stuck I'm willing to try and help, if needed. No pressure, just an open ended offering.