Intel SRT vs Superfetch

So far, I see these advantages for Intel SRT:

Cache will remain after boot
Possibly faster boot time

Yes, that's correct. It's similar to running a solid-state drive all of the time (but not exact), so the applications you use most will launch a lot faster (since they're cached on the SSD).

For Superfetch:

If needed, RAM can be used for something else
Way faster
Can be used to cache more than one spinners.

Yes, but superfetch doesn't cache everything in memory. It also stores a lot of stuff on the hard drive itself, in locations it determines to be the quickest. At the current moment in time, unless you really need more then 8GB of RAM (and you know if you do), then you would most likely see a much faster overall system using the solid state drive. Applications don't usually take a lot of memory except in certain cases (e.g. gaming, video editing, file compression), and even then, it's rare to actually need 16+ GB of memory.

In my case, reboot very rarely so I am leaning towards more RAM. Anything I'm missing?

It's not only rebooting you're saving time with using SRT, but application loading times and write speeds (depending on the mode you use, enhanced or maximum). SRT can allow the use of the SSD as a "write through" or "write back", so keep that in mind.

Lastly, you might also want to have a look at the following question: What are the pros and cons of a solid-state drive? It covers a lot more material then is relevant here, but it may help your decision.