Both shows are pretty much about a bunch of people separated from the outside world by supernatural forces. So why should one watch Siberia over UTD?
1. You'll get closure. As far as we know, Siberia hasn't been renewed for Season 2. And based on the way ratings are going, the chances for renewal are unlikely. This all but assures we'll get some finality by the end of this season. Whether they're all eaten by monsters or only a handful get home, at least we'll know they won't be running around in the Siberian wilderness next year. The same can't be said for the residents of Chester's Mill who will continually be trapped Under the Dome until the ratings take a dive. It's a sad day when the ratings dictate the story and not the other way around.
2. You won't feel like shouting at the screen every few minutes telling them to get out already. In Siberia, the people isolated themselves voluntarily, in the hope of winning some cash. Any weird and creepy thing they experience can all be chalked up to the producers messing with them trying to get them to leave. This explains why they may ignore or give less importance to the weird/creepy things around them. It also gives us a reason on why they're not in a hurry to leave. The same can't be said for the characters trapped Under the Dome. A freaking dome fell and it's no big deal to them. The adults ate in the diner like usual. The kids threw parties like usual. Even the delivering of appliances proceeded on schedule. The most anyone has attempted to get out of the dome is to gently touch the dome. Literally. The only exceptions are the psychos of the show. The crazed deputy who tried firing at the dome and Junior who went deep into the tunnels and when confronted by the dome, proceeded to punch and kick the dome in frustration.
3. The girls are badasses in Siberia. Siberia has an archer who eats deer for breakfast. Another one poisons her fellow contestant, sleeps with then blackmails the cameraman just to get ahead. Another burns down the entire camp food supply. UTD has Sheriff Linda.
4. There are more mysteries in Siberia. There are large monsters eating people, "Others", green skies and time travel/time convergence/time loop possibilities. Aside from the awesome dome (and its offspring), and some Gangnam-style seizures, nothing much else is happening under the dome.
5. Siberia isn't being reviewed by TV.com. You can miss out on episodes of Under the Dome because Tim will always be there to give a great recap and review of the show. The comments will fill you in on everything else. The same cannot be said for Siberia.
6. and 7. Carolina and Esther. Just some of the badass eye candy you'll see in the show.