We go with option D ;-)
The answer here is not cut and dry.
If you got a smaller HD and didn't need a lot of storage and you only wanted speed (web server). Then swapping out the HD with a SSD makes sense.
If you need more storage then often one just upgrades the HD to a larger size. Or adding a second HD drive for still more storage.
In some cases people want there cake and eat it too! So you end up either putting in a SSHD gaining the storage of a HD with the performance gains of a SSD. Or, doing a dual drive setup. Here you stay with the HD you have and add in a SSD drive. Then you can setup Fusion or run the drives discreetly.
The only issue you really face here is the loss of your optical drive if your system had one internally. If you have a newer model you might encounter issues with the second SATA port as some models have an issue with the SATA I/O speed.
We gave up doing the dual drive HD/SSD setups. Instead we are only using the hybrid SSHD drive for the first drive. We also only use SATA II (3.0 Gb.s) HD drives in place of the optical drive due to the system port issue.