I say watch both but that's just me.
B5 and DSN do share some similartites in that they are set on a space station and feature a war. But you can enjoy both. B5 does have an even more serialised element to it than DSN, if you do watch B5 bear in mind S1 is more about setting up the universe and doesn't show B5 at it's best and there area fair few weak episode in there. But once it gets into S2 andesp. S3-4 it is very good.
SG-1 does have a certain amount of humour in it, and it is semi-serialised like B5 and DSN. And once again S1 is weak in the begining but definatly ramps up a notch towards the end of the season.