In general you'll find you probably need both - while
assemb.ly might be slightly shorter, explaining to people that it's "assemb dot el why" will probably take longer than
Many of the brands that make use of this particular TLD do so as a URL shortening process, but they then tend to resolve on a more common TLD, for example the Bitly domain
bit.ly redirects to bitly.com, similarly
ow.ly for hootsuite.com and so on.
Note also that you may well fall under far more stringent content regulation if that is your primary domain - this article from 2010 talks about the case of a domain being seized by NIC.ly because
the content of our website, in their opinion, fell outside of Libyan Islamic/Sharia Law
As has been noted, the .ly TLD is considered a "country code TLD" (it's not on Google's list of Geotargetable domains), and so will probably rank higher when searching from within Libya than for people outside of it. Your best hope is that you don't use a CDN, and host the servers in your local country so that Google can use the IP address to help localise you.