Should i start a DK right when i hit 55 or keep going with my shaman.

keep going with your shaman even though DK's are decent and exceedingly easy to level up the fact is many people have them and it will be much harder for you to find groups later on where as if you are a shaman you can be elemental DPS and a healer and you will be one of the most needed

Both Shaman and DK are not particuarly powerful at the moement.
I would say keep going with the Shaman. Hybrid classes change a lot due to patches and can often be kept alive just because of that.
Furthermore there are a lot of deathnights.
If you don't go tank as a Deathknight then your melee dps which is not as desired as the Shaman ranged dps.
Tanking is kinda hard now.
But its your choice.

i would have to say go with the shaman :P i always found the shaman as a fun class

Well I would wait. Depends on what you want to do. I just kept going and got 2 characters to 80 before I started a DK (Catacalysm wasn't released). But if you start to tire of leveling your shaman, which you probably shouldn't since they're so much fun xD, but anyway go ahead and start a DK if you get sick of leveling your shaman.

I say stick with your shaman cause I find DK's boring but thats my personal opinion. The first answer says they both suck at the moment, that is not true at all. I have seen many DK's pull 22k DPS at 85. So DK's are very powerful. DK tanking is the easiest thing to do. If you go Shaman, then I say go ele/resto (dual spec) since they share gear and it will decrease your dungeon que times. And if you tire of both of those classes you can always make a rogue or druid like me =)