Although a lot of I/O will shorten the lifespan of an SSD, the durability has improved a lot in recent years. A lot of manufacturers are now offering five year warranties on their SSDs. I only have one BOINC on one of my SSDs, and it doesn't run a whole lot since it's my tablet.

Ssd all the way.
I could never go back to a mechanical drive as my main OS and on my main computer.
Intel SSd here, 4 years old pushing 32000 hours. Nothing special done to preserve it - none of the changing of the pagefile, etc. Can get the actual data that's been pushed through it but I'm happy. I look forward to PCE SSD when I get one.

Durability of SSD depends on how much you write on it. For example my Samsung 850 EVO 250 GB has a guaranteed lifetime of 40 TB written if I remember correctly. So that's about 20GB per day if you expect a 5 year lifetime. Other manufacturers in that price segment have comparable lifetime, if you pay more (for example the Samsung 850 PRO) you get a higher lifetime.

Data written by WCG depends very much on project. For example the Clean Energy Project used to write large amounts of data to the disk.
To lower the amount of data written by WCG, you can specify in the BOINC manager settings, how often a project should write to the disk. I have this value set to 600 seconds, which is low enough to lose not too much work if you switch off the PC, but high enough to lower the amount of data written (I think standard value is 120 or 180 seconds?).