Elric is a genius when it comes to using alchemy, especially in combat, and is quite skilled with analyzing his opponent during combat and developing a strategy on the fly.
While Cullen may be several times Elric's age, he certainly wasn't fighting the entire time, so he shouldn't have more combat experience then Elric.
Also, it seems that Cullen only really stood out for his speed in the Twilight universe, but even with equal speed in this fight, they were about the same speed anyway, with Elric being slightly faster with the lighter automail.
Even if Cullen could read Elric's mind to know what Elric is going to do, he has no experience fighting opponents that use the earth in combat like Elric does.
Cullen, based on Patterson's portrayal of him, is 14" taller than Elric (Patterson is 6' 1", Elric is 4' 11"). As Elric told Pride in their fight, he is used to being the smaller combatant in a fight, so fighting with Cullen should be no different.
Elric received martial arts training from a young age, allowing him to be proficient with his arm blade and a spear, which lets him keep Cullen at bay. Cullen also never really displays actual martial arts when in combat.
Finally, if Elric uses the Ultimate Shield technique on his arm, not only can he essentially block any blow Cullen can dish out, but he can slice Cullen to ribbons since he can slice stone and steel apart in those circumstances. Not to mention if Elric uses Scar's Destruction technique, he can end the fight in one well-placed blow.
Voting Edward Elric for superior intellect, training, and techniques.