I think the student(s) that have had the biggest impact on the team (have lead the most, helped out the most, inspired the most, grown the most etc.) should always be the ones nominated for the award.
I can understand the reasoning behind giving preference to a junior over a senior... To a junior being a Dean's List Nominee, Finalist or Winner is something they can put on their college apps... To a senior the award won't have as much "use" - for summer internships it'd probably be meaningful, but come time for sending out resumes for jobs after college it won't have the same weight. As much as that "usefulness" exists, I can't help but feel that a team's superstar(s) have to be the ones nominated, regardless of other factors...
For a senior, this is their last chance. Odds are good that since they're a team's superstar they've had a growing role over the years - they were probably a great leader and inspiration in their junior year... maybe they were nominated and perhaps were a finalist and/or winner. Since they have such a love for FIRST, they're probably even more involved this year than last year (their junior year)! So nominate them again - maybe they'll have grown enough to be a winner this year!
Since the award is now in its third year, there's really no excuse to base the nomination indiscriminately off of age. If your team's superstar is a junior or senior, nominate them... even if they're a sophomore or freshman, I'd nominate them (although I honestly would be surprised if many teams exist where their two superstars are sophomores or freshmen). I really don't think preference should be given to juniors or seniors.
Nathan
P.S. FIRST's decision to give preference to juniors in the judging frustrates me... Perhaps I'm just being cynical, but I think part of their reasoning is so that a winning student's positive image feeds back to them... colleges seeing that these fine students are associated with FIRST adds to FIRST's reputation also. If they just wanted juniors to have the benefit of throwing it on their college app, they'd simply point out that juniors have that added use of the award. I don't think FIRST needs to be casting that doubt on whether the award winners are truly the most qualified...