SATA II is 3Gb/s i.e up to 300MB/s
SATA III is 6Gb/s i.e up to 600MB/s
In practice the SSDs don't get to operate quite at that speed. Depending on the model, manufacturers will typically cite speeds for most current better drives of between 500MB/s and 550MB/s with variations for read and write. You can get cheaper drives that show wide differences between read and write. Of course these are maximums and other system factors can get in the way.
Sandisk (the first google I found, not necessarily a recommendation) suggest
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Originally Posted by sandisk
SanDisk Extreme SSD, which supports SATA 6Gb/s interface and when connected to SATA 6Gb/s port, can reach up to 550/520MB/s sequential read and sequential write speed rates respectively. However, when the drive is connected to SATA 3 Gb/s port, it can reach up to 285/275MB/s sequential read and sequential write speed rates respectively.
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Hard drives will have a wider variation in speed, both read and write, and even access time. The moving parts get in the way and obviously if data is scattered on the drive it's going to make hard work of it. Theoretically speeds of 50MB/s to even 120MB/s might be achieved.
I did have a HDD, only 5400 rpm in my 2010 lappy. To give it a bit more potency I put a basic SSD in it. There was a noticeable difference in boot up times, but otherwise, for the stuff that I use that for, the SSD is of little benefit.