Field Spell vs MST

Not quite clear on this. I have a Field Spell on my field. My opponent plays his own Field Spell, to which I chain Mystical Space Typhoon. I know the rule where only one field spell can only be active at once, but if I chain MST which destryes his field spell. I know it doesnt negate the eff, but what happens do I lose my field spell and what happens to his field spells eff?

The opponent's field spells is not considered to be "on the field" until it fully resolves. So, when you chain MST to its activation, it never actually hits the field and your field spell stays.

Ok thank for the help :D