My take on the 3 realms:
Syrtis (My own)
Poor teamwork, but some good players. Works largely by good players forming teams amongst themselves and everyone else joining. When good players fail to form teams realm can be boring. Realm not particularly strategic (I am being kind here - standard strategy is 'lets kill something, CHARGE'). Can be fun through
Ignis
Under populated. Teamwork is either crazy good or terrible. No idea why. Some very good players there. When they are in the mood they are mega fun to fight against, and I suspect with.
Alsius
Teamwork almost always good, great strategic players too. But they play safe and are kind of boring.
I'd say go with Syrtis of Ignis, unless you simply want to follow the crowd in which case go to Alsius.
Re classes:
Same as always:
Charge and do or die type - choose barb
Long fights/protect others type - choose knight
Heal others but dependent on them for safety - choose conju
Sneaky, cheeky beggar who can't kill but can really mess things up for enemies - choose lock
Sneaky, cheeky beggar who can barely kill, can barely mess things up for enemies, but has very high annoying factor for enemies - choose hunter
Stand back and safe and kill things - choose Marx