yeah i got the same question also if they read or write slow how can sata III still have 3 gbs or higher compared to sata II with faster read and write? what i am i missing but to your question i would think read would be more important than write i believe your computer sits there and reads more than writes unless you do some heavy computing.
For a caching specific drive, I would go with 1) quickest access times and 2) higher writes over read. When you are caching a drive, you are writing to the cache drive to hold until utilized by the memory or frequently used, which access times help. Most of all SSD's have really good read speeds, so I won't be to concern with them.
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.