An SSD is faster, both in terms of access time and transfer rate. The SSD should also be more reliable, although it does have a limited lifetime depending on how much you write to it. Intel claims their SSDs will last at least 5 years if you write 20GB to them every day.
Which is a smarter buy depends on your needs. The biggest drawback to SSDs is their higher cost and lower capacity. Most people who buy SSDs compromise and buy a relatively small one to hold the operating system and then also buy a much larger hard drive to hold the bulk of their data.