Heh okay, emacs is extremely programmable. I don't really consider it an IDE but that's fair.
It'd take a long time to explain the differences but emacs is far from lightweight, it's a beast. LT is very barebones. And LT is written in HTML/CSS/CLJS which makes it stylable and programmable as such, a real advantage over emacs.
Emacs is an IDE, first and foremost, to Emacs Lisp itself. Other Lisps are close. It's an excellent editors for other languages, and a good enough IDE for some languages. See my C/C++ guide in Emacs: http://tuhdo.github.io/c-ide.html.
Fyi, people wrote parsers, chat and email clients in Emacs that LT is still dreaming.