The point of a cache is to keep a copy of data on the SSD so you don't have to wait to read it from the HDD. That strategy only works if you're able to get data into the cache that is then read many times. If each block of data that you cache only gets used once, then there's no performance improvement at all, since you have to wait the full amount of time to read it from the hard drive in order to get it into the cache in the first place.
So in a well functioning cache, you'll be reading from the SSD a lot more than you'll be writing to it. Given that, I would suggest that read speed is more important than write speed.