I was tempted to say Magneto right away, but the more I think about it, the more I think Sinister would win. The guy has several different powers to use at his disposal, including high-end telepathy.
Mister Sinister's most deadly weapon, in my opinion, is his telepathic powers. Which is great for Magneto for two reasons. The first is that Magneto has shown the ability to dampen telepathic abilities by the manipulation of electromagnetic forces. The second reason is that Magneto is inherently resistant to telepathy (Though the cause of this is up for debate- 1. latent telepathic powers of his own, as suggested by Professor Xavier during the Secret Wars, and his apparent ability of astral projection. 2. An exeptional willpower. Which would be very exeptional considering he fights while under psychic assault by both Jean Grey and Professor Xavier simultaneously.)
Mister Sinister's other weapons against Magneto would include energy blasts and using his unusual malleable form to create weapons from his body. Both of which shouldn't make it past his forcefield.
Now, we see why Mister Sinister would have tremendous difficulty harming Magneto, let's go over some of the weapons Magneto has to win this fight.
We have the usual argument- environment. Anything that Magneto can control and use as a weapon. Good but I'd say this is an iffy tactic considering Mister Sinister's malleable form would offer him a degree of resistance to such physical assaults.
Personally, in this scenario, I like the idea of lightning (Or electrical blasts, if you prefer) or (More fun yet!) microwaves. I always think of plastic when I see Mister Sinister contort his body, and I think we are all familiar with the dangers microwaves pose to plastic. (Assuming their physical similarities account for much)
Other options include electromagnetic blasts and (lame) battlefield removal with electromagnetic force bubbles.
Another interesting tactic Magneto could take advantage of is invisibility as demonstrated when Magneto flew up to the moon to tell Scarlet Witch and Quicksilver that he was their father.
To give this fight a meaningless rating out of 10, I favor Magneto 8/10 at least.
Addition after seeing people posted while I was writing this out: Magneto's helmet, in the comics, is not the source of his telepathic resistance.
__________________ "The weakest dogs bark the loudest."
__________________ Harry Callahan: Well, when an adult male is chasing a female with intent to commit rape, I shoot the bastard. That's my policy.
The Mayor: Intent? How did you establish that?
Harry Callahan: When a man is chasing a woman through an alley with a butcher's knife and a hard-on, I figure he isn't out collecting for the Red Cross!