I handled the UDP when it debuted at SHOT 2015 and my thoughts both then and now were/are "meh - just another pistol-calibre carbine". Sure, it's a bit more refined and streamlined than an AR-pattern 9mm with its reduced magwell dimensions, but at the end of the day the UDP is just a closed-bolt, straight blow-back design. More same-old, same old, and not terribly inspiring nor innovative.
Say what you want about the Kriss Vector, but at least it is unique. The recoil-reducing "Super V" design is intriguing, as is the notion that the Kriss was the first carbine to employ the ubiquitous Glock mag. Whereas the UDP is at this point still a "boutique" gun, the Vector has found limited acceptance with Forumvarious law enforcement agencies.
The above is simply how I view the UDP9 and the Kriss Vector in isolation, and is why when it came down to the same choice I bought a Gen 2 Vector SBR with the factory folding stock on the EE an few weeks back. That said, I am convinced that there are better choices than either the UDP or the Vector. The B&T TP9 springs immediately to mind as one such firearm. As does the AR carbine, or (for those who can own them), any of the proven Gen 3 SMG-based carbines such as the HK94, Mini Uzi, etc.
But if it comes down to a choice between the UDP9 or Kriss Vector? The latter wins hands-down for being both different and proven. YMMV, of course...