Everything I type is based off my experience. I've played through Northumbria 3 times and Wessex once. You might want to put your reading glasses on...
1) The ending with Northumbria is exactly the same as it is with Wessex with the exception that the Danish win the Battle of Dour-Enz (the naval battle where Bodo dies) and the big fight right before you take on Sven for the last time. I've played through with Northumbria only losing Reginhard and Bodo (everybody else stayed). I've played through Wessex losing only Egil (the king's bastard), Reginhard (seriously, I hate that guy), Asbjorn (the Assembly judge/doctor) and Bodo (I'm pretty sure he is scripted to die no matter what side you choose in the storyline). The burning mead hall is agitating but if nothing could kill you suddenly while playing Wessex then you have nothing to worry about playing with Northumbria.
2) I prefer Northumbria solely for the superior troops and the fact that I can ultimately keep more companions. The only companion you can lose (at least the way I did it) joining Northumbria is Bodo. Everybody else can be talked into staying with you. After joining with Northumbria you take Dun Taro in the far North of Scotland; the idea is that the Kingdom of Alban will attack that fort in an attempt to take it back instead of attacking anything else belonging to Northumbria (and surprisingly it works). The storyline with Northumbria is exactly the same as it is with Wessex; take fort, defend priest, naval battle, field battle, kill Sven, end game. I had a hell of a time on the last field battle between Wessex and Northumbria because of troop fatigue (I still don't have that down yet). I prefer Northumbria simply because of their territorial holdings at the end of the storyline (they hold almost half of England). However, because of their size a lot of factions will shortly declare war on Northumbria after the end game. In my opinion the Kingdom of Alban (Scotland) is easily crushed. Wessex can be dealt a harsh blow if you go take some towns/forts immediately after the hellish field battle (I can't remember the name of the battle for the life of me). I try to finish off the Kingdoms of Alt Cult (because they border Northumbria along its entire Western border which makes it easy for them to raid/invade once the war starts) and East Engle (because they are practically entirely defeated at the end of the storyline) after the end of the storyline. I have no idea about rewards.
tl:dr The size of Northumbria at the end of the game ensures near-immediate wars after you complete the story. However, because Northumbria fields Norse troops, nobody really ultimately stands a chance against you.
3) a) If you are siding with Northumbria: kill Reginhard in the Holmgang, when you go to turn the letter over to Northumbria the only person that didn't say "I'll follow you to the end" was Brunhild and she says something like, "If you turn that letter over to Northumbria then I'll..." and I respond with, "WHAT?! WHAT WILL YOU DO?!" and with that you put her in her place and she says something like, "I don't know. We've been through too much to go our separate ways now." I bought everybody's good favour though (all relations at 100; I generally have money to blow for stupid things). The only companion I lose when turning over the letter to Northumbria is Bodo.
b) If you are siding with Wessex: I really have no clue here. I've played through with Wessex once and when the time came to decide to turn the letter over to Wessex, only Asbjorn (he's the judge from the Assembly that joins you) and Egil left my party. Egil thought it would be a good idea to play me close and try to test my cool and he ended up occupying a plot in the earth. He was upset that I was joining Wessex. I probably could have saved Reginhard if I challenged Egil to the holmgang during the Assembly instead of Reginhard, but I despise Reginhard so he is going to die in the holmgang no matter what.
tl;dr I lose more companions siding with Wessex.