User blog comment:Dj0z/Dj0z's Assassin Nocturne Guide/@comment-5045559-20120526080345

After a few tests, it seems clear that by picking CV+Smite (which both have the lowest summoner spell cooldowns btw), the early game is sufficiently faster to allow more deadly ganks, which in turn give a higher chance of snowballing. If you go as fast as possible and CV/ward properly against counterganking by mid or jungler, you will probably not regret Flash because the of the faster farm which gives you boots sooner. With all the movement speed included in the build, you effectively have like 394 MS with T1 boots, which is faster than the typical AD carry with Zerks and most other rangeds with T2 boots (except Mobility).

Of course it also saves a few but useful seconds when stealing their red or blue, and further lowers the chance of ninja long-range steals on Dragon/Baron.

With this even faster farming speed, i've used a slightly modified version of the build where i get cloth 5pots -> boots ->Wriggles rightaway -> Rush Youmuu, effectively delaying the Zeal because early Youmuu's ArPen is more effective, and that if you land Q, activating Youmuu will still let you catch people with T2 boots easily.

The downside of course, is that you cannot Flash-kill anymore and if you're focused in a badly engaged teamfight or countergank, there's no way out anymore.

TL;DR: CV+Smite is easier and faster for earlygame, you want to snowball in farm and/or champion kills as early as possible, but you're more vulnerable later on and people might escape from you with Flash when ulti/CV are on cooldown.