I had to do a bit of research into Tails' abilities to answer this question myself. I know that Tails is capable of flight by spinning his two tails like a helicopter blade. Magneto is capable of flight by using his power on the metal portions of his costume. So they can easily match each other in the air.Sstavix, I need your advice on deciding Tails vs Magneto. I don't know enough about either character to make a good judgement.
But Tails lacks pretty much any offensive power whatsoever. Apparently, he's capable of tinkering with machines and has used them in some of the games, but we know that that is largely ineffective against Magneto. (Fun fact: Even though he's been called the "Master of Magnetism," Magneto's been able to use his mutant power to manipulate any sort of metal, including non-ferrous ones!) Magneto would be more than capable of pelting Tails with whatever metal he can attract - possibly from Tails' own disassembled mech - until the twin-tailed fox eventually grew too tired to dodge any more. It would be a long, but one-sided fight with an inevitable conclusion.