Gabbard has an excellent shot at statewide office soon. Hawaii is a small state, so there will be less competition for likely openings. Granted, it's a state that has elected elderly officeholders a lot, so while Neil Abercrombie is probably going to end his career after his next term as Governor (assuming he wins) Mazie Hirono might stick around in the Senate for a long long time. She has the background and progressive record to run a credible grassroots national campaign. If she has national aspirations, Washington may be better than being Governor of a group of islands in a different time zone than the rest of the country.
Polis has a bit more trouble. Colorado is fairly conservative, so statewide office may not be his best bet (although Tammy Baldwin's election in Wisconsin suggests it's a valid route.) On the other hand, Democrats are expected to be the minority in the US House for a while, which may make other positions more attractive. But those might not open. Bennet's probably going to stick in the Senate for some time, and while Udall will be seventy in 2020, 70-year old Senators tend to run for reelection. Things can change. I've argued that Bennet's a great pick for Veep, in which case there may be an open seat in 2016. Udall might lose, an opportunity for a Democratic challenger in 2020. And there should be an open primary for Governor in 2018.
If Bennet or Udall end up losing a Senate reelection or end up as President (unlikely) or Vice President, then Jared Polis could probably advance to the Senate. However, if Gardner wins this year, Polis' best hope would be that Clinton opts for Bennet as her running mate (as you mentioned, Mister Mets), because I think Gardner seems like he would be a very difficult incumbent to dislodge, along the lines of Sens. Sherrod Brown (D-OH) or Mark Warner (D-VA).
Tulsi Gabbard should definitely consider running for Governor of Hawaii if she wants to advance at the state level, since HI has two Democratic incumbent senators, and probably will for a very long time, but if Schatz retires to run for Governor in 2018(which seems unlikely), Gabbard could run for that Senate seat in the 2016 election, and she might end up being the youngest senator in the body, which would give her plenty of time to be a running mate or run for the Presidency herself. Gabbard might want to move up in the House leadership if she doesn't view any openings in Hawaii, and she could advance quickly if she proves her worth.
Between the two, I view Gabbard as having the brighter political future, but I wouldn't be surprised to see either on a national ticket in the next 10-20 years.
