I'm a dunmer assassin, working for the DB, Companions and Thieves guild. I don't use much magic to be honest, the occasional muffle maybe - the rest comes from potions, like paralysis and invisibility.

I like to sneak and stab/shoot with the bow a lot, but I understand neither of the 2 transformations are really sneaky. I have like 150 magicka, don't know if that's really good enough for a vampire lord.

As a character, I am a killer for hire but not really without emotion, you could say I'm lawful in a sense.

First off, welcome to the Forums.

It really is a matter of preference.

Staying normal is good in most cases, you retain your ability for rested bonuses.

Werewolf for a stealthy class is only if you want to be able to transform into a big raging monster and frontal attack your enemies. (Typically would involve finding safety) I personally find Werewolves fairly weak later on (unless you have Dawnguard which I know little about)

Vampire (asssuming you do not have Dawnguard) would be your best bet if any for a affliction. Vampires actually do have a 25% bonus to sneaking, they also get a bonus to Illusion spells if you start using magic more. You get some other bonuses (A once a day invisibility power at Lv 4 Vampirism)

As for If you have DG... Well I wouldn't know, though I'd assume the effects above (atleast sneak and illusion) still apply.

Hey, thanks for the reply and welcome.

I have all DLC and I'm playing on Master but with a lot of mods that make it harder (deadly combat for one).

To be honest, even though I'm using a mod that rebalances stealth skills and makes sneaking harder, I'm already getting barely noticed.. and I have 1 point in the base sneak perk.

First welcome.

Now honestly the only reason I even responded to this is because we are playing the exact same character. Dark elf, same guilds(minus the companions as they didn't fit for me) sneak, bow etc.

So I guess you have Dawn Guard? I actually still only have 100 for magica and chose to be a vamp lord. I think this would be your best bet out of the choices you have up in your post.
The werewolf doesn't fit your build IMO. I think being a vamp should be with the sneaky archer type of character which you have. I think the werewolf best fits the more "knight" type of build if that makes sense.

I do have to be honest though, as I used the advantages of being a vamp lord a great deal for the duration of DG I am now noticing that I hardly use the benefits of being a vamp now that I'm back in the regular game. I just kind of lost the desire to play as one and tbh the side effects like the negative effects on your stats are really starting to annoy me. So much so that I'm greatly considering getting rid of the vamparism. Which is not that hard at all.

So I would say ask yourself these 2 questions:
1. Do you think you would use/ benefit from the abilities of vamparism more than they would hinder you?

2. Do you think you would stuck with it for the duration of your character?

If you answer yes to both then I say go for it. But if your really not sure then I would say take a moment and really think if its something you want to go through with or are you simply curious as to what it's like being able to transform?