Horses are a lot easier to train then cows.
I think it depends on who is working with the animals. Cows (females) are not oxen (aged steers), have quite different temperments and acceptance levels in situations. Cows are not usually driven, labor takes away from their milk production, the main "purpose" for being kept. Being tied to a load, oxen usually are less likely to run far or very fast, which is unlike driven horses.
Having had both horses and cattle (though we didn't drive the cattle), they can both be trained, just takes different techiniques and your choice of how hard to work with each species.
Our 4-H cattle were very easy to handle, since they got handled DAILY. Led all over, brushed, hooves cleaned and trimmed just standing. They got tied to stand or practiced loading in the stock trailer. Cattle loaded as nice as our horses do! All come when called, are very cooperative. Training takes time and dedication, but is not really hard. Working with them keeps them coopertive, thinking YOU are the Alpha of the herd, needs to be consistant handling and done daily.
Our horses are NICE horses, very easy to handle and accepting of new things, good workers ridden or driven. We work at keeping them fun to use, so our standards of expectation are quite high. Again, not hard to train, just takes that daily handling.
So I would never discount how capable cattle could be for the pioneers of the past. They were the most common draft animal for moving westward with the family covered wagons. Freighters went to the equines (mules and horses) for speed, made more money getting loads delivered faster. Cattle STILL are the most common draft animal in most undeveloped nations, for many good reasons.