Simply put, you're more likely to get Shadow Tech, and you're more likely to be successful in integrating it into your own tech.
Consider that the Vorlons actively try to hide their level of tech from the rest of the Universe, and routinely hinder attempts to achieve higher technologies that they aren't "ready" for.
I'm pretty sure that the reason why Vorlon tech only had to be reverse engineered because 1) No Vorlon were around to help integrate the tech and 2) If a Vorlon was around, he'd fry your arse for trying.
The Shadows, on the other hand, would likely have handed you Shadow Tech with a grin, a nudge, and a wink (And probably an instruction manual to boot), and set you off to wreak havoc on your enemies (And while you're not looking, do the same with your enemies!)
In terms of the two races and their ships, specifically, I'd still have to stand by the Shadows. Their superior maneuverability outmatches Vorlon survivability. Though, in terms of "coolness," while they are both very closely matched, I must side with the Vorlon. Standing there and being cryptic is just cool.