I view it this way;
Stalin tried to make Finland a part of the Soviet Union.
He failed.
This equals Finnish success in a struggle between Mannerheim and Stalin.
The Russians backed off and gave up. I interpret this as a decisive Finnish victory.
What do you wish to call it?
Like I said, a victory in which you give up 15% of your territory, including your second-largest city, 30% of your industrial capacity, a third of your hydroelectric capacity, 75 locomotives, 2000 rails cars, render over 100,000 refugees from the lands handed over as refugees and are forced to lease other bases to your enemy seems an odd kind of "decisive victory."
What do you wish to call it?
A negotiated surrender with something better than unconditional terms.
The fact is that after their initial humiliations, the Soviets reorganized and reinforced their forces. They began punching holes in the Mannerheim line. The Finns, running out of men and ammunition, saw the writing on the wall and asked for terms. And while the Soviets made noise about conquering all of Finland, what they settled for was more than what they'd been demanding during the October, 1939 negotiations, terms the Finns rejected at that time. So if you want spin that conceding to terms more harsh than those you'd rejected four months earlier constitutes a decisive victory, be my guest.