Many (majorty?) celtic longswords are completely unsuitable for stabbing due to rounded tip.

Roman scuttum+gladius combination allowed forming sth like phalanx with the line of closely packed shields. the result was that each solider was very well protected and can reach his enemy almost without exposing himself to attack. You can also slash with gladius - Livy mention even cut off limbs as the wounds romans inflicted on macedonians in early stages of II macedonian war.

On the other hand, if you use slashing longsword you need more free space to allow blade get momentum. Without momentum (for ex if gaul champions penetrated roman line and were swarmed by legionares) the longsword loses almost all its merit.

So in single combat I'd place my bet on celt but in formation with experience equal I'd choose romans