Sinister may be evil knowldge. He knows where he will be in his own future, he also knows where his opponent will be. This isn't the same as omnicient, it is just Knowldge. From our own perspecitve it is just a bit less limited than present. It makes him an able planner, he can engage in battles that he knows he will emerge form and reap the rewards. He can thus fight anyone and get the better of them and himself. It is in his own intrest to fight, win or lose. If he is this evil knowledge that he portrayed on his introduction to the X-men, then he can locate things that are alone and forgotten and make alligences or attain new powers.
He is probely trying to bring about a dark age like the one of our own past where records are no longer kept. Then he would have the best access to goods.
He would prob. dampen Magnetos' power effectivly shutting him off.
This battle would not end. Period. Magneto doesn't possess the power to destroy Sinister (Well, for the most part), and Sinister would only win if he managed to nail Mags with some nice energy blasts, or TK/TP...
For people who don't know anything about Sinister,
Sinister is from the 17th (Or 18th? Or 19th? Long ass time ago, I always forget the date, I'm almost certain it was 1800s) he was born a human, named Nathaniel Essex, and had a wife and kid(s? I forget, stupid useless details) He was really into genetics at the time, and was way ahead of anyone else in his field. Eventually he grew distant from his family and they seperated, Apoc met him and gave him the base of his powers. Sinister honed his powers and mastered them, he became in essence a toned down Apoc. Where Apoc can rearrange himself to copy/create any mutant or physical property, Sinister can rearrange his physical makeup on a molecular level at whim. I.E. he can be blown to shreds and pull himself back together, change his appearance, body size/shape, etc. He has EXTREMELY high levels of power, being able to telepathicly kick the crap out of psylocke, fight off Storm, Iceman, Dazzler and Marvel girl all at the same time. He was frozen solid by Iceman and broke free almost instantly (A feat Grey Hulk took quite a while to perform) He's destroyed the X-mansion in one shot, and is overall just plain 'ole crazy.
Now, Mags has more power in his single mutant ability, which is extremely versatile, but Sinister is slightly weaker and possess many more abilities. In the end, it all comes down to who the writers are, the context, and pretty much the story in general. This can go to either one, IMO.