wow, 1/20? Well the last 20+ tiems i've commed ns_dig_siege_b3 I've won, if you dont get a newb commander you can win very very very easily.
Lets look at the stats:
Marine Wins: 419
Alien Wins: 438
Odds of marines winning: 19:20 (this is alot better than 1:20 as said above)
Probability of marines winning: 48.89% (this is alot better than the 9% or 4% as given above [1/20 and 1/10])
Seems pretty Even to me, its almost a 50-50 chance, statics show that the map is even and not "1/20", the aliens only outscore the marines by 11,027 Kills it may seem alot but check out other maps:
Map - Kills/Team Difference
ns_stargate-sg1-beta4 - 25,009
ns_the_alien_breeder_v2 - 19,514