Sonic X- Lightning timer(mach 300+) Sonic
or
TMNT- 4 Hypersonic(mach 5-10) Turtles

this is too cruel for a vs.

So he obviously has superior speed, doesn't make much of a difference when the Turtles are fighting in the mystical plane here instead of the physical plane, each Turtle with building level DC. (I believe Sonic X Sonic was also building level, if I remember correctly)

But then again, Sonic has moon level durability since he took a full power punch from Knuckles, who moved the moon with a punch. Regardless of Sonic's superior speed, I still think the Mystical Turtles would win because of their superior strength, agility, skill, and maybe even stamina. Sonic has the advantage in terms of speed, but...... I think that's just about it.

The Mystic Turtles also have planet level durability since they tanked attacks from the Tengu Shredder, who was a planet buster. (This was not only implied, but he was slowly reality warping the planet, and it was stated that he would he was merely "playing games" with every one, implying that he wasn't really trying to reality warp it as fast he could. He was in no rush.)

He also, in his dragon form, EASILY broke through the barriers of three keystones that contained and channeled the mystical energies of the planet, destroying them.

If the Tengu Shredder WAS a planet buster, then why wasn't the planet on the verge of blowing up when he fought? Attack potency, my friend. Not everything necessarily needs to have a big explosion. Example; Molecule Man shot the Beyonder with a blast capable of shattering several billion dimensions yet it didn't level the building they were in.

Class 100+ for each Turtle both with and without power-scaling, (It was stated that in this form, they can move mountains)

I think this is overall, a very good fight considering what Sonic has shown, and it'll be a very long one if you count Sonic's insane durability, but nonetheless I'm still leaning towards the Mystic TMNT.