In a battle between the two, instant speed would have a better chance of winning.
The reason is because speed is a ratio, meaning you can be in continuous motion, i.e. a constant rate of movement[\B]. Consistent physical movement; your location is constantly changing.
The ability to open an "instant portal" is just that. You can instantly create a portal at a specific point or make two points the same. The problem is that the scope of this ability is limited first and foremost by the points. This is the flaw that someone with instant physical movement of theif entire body can exploit. In order to "teleport" a moving object with a portal, the portal would have to be converged with the exact location of your target, a feat that would be easier said than done if the target is in instant, constant motion, meaning the target will have changed location (moved to a new point) at the same time you set a portal at the target's previous location. In other words, when you instantly set a portal at point A, your target with instant speed is instantly moved to point B at the same time, making your portal useless.
You would have to set a portal where your target is going to be in order to catch them in the portal, but then you would be relying on your reflexes to set the portal in the right place in time to catch your target and without your target being able to avoid your portal, all while hoping you can do so before your instantly moving opponent is able to get to you first. And even if you manage to catch your opponent with the instant portal, the speed of your portal's teleportation rate would have to be fast enough to teleport your opponent before it can change location (keep in mind it's moving at "instant" speed according to the rules), as being able to spawn a portal instantly isn't the same as being able to teleport something instantly.
Personally if it's a battle between the two I'd choose speed, though I'd rather have the portal outside of a fight. In a fight against the two skills though, you'd have less variables working against you with speed.