Should the pagefile be placed on the SSD or the HDD?

Solution 1:

You have two options really:

1. Put it on the SSD for better performance from your page file. This will DRAMATICALLY shorten the lifespan of you SSD if you are frequently writing/reading from your pagefile.
2. Based on the amount of RAM you're running, you could potentially disable the pagefile in its entirety (or just hide it on the HDD).

I'm running Win 7 Ultimate with 12GB of RAM, frequently gaming, photoshop, etc and I never hit 12GB. But even if I do, I tossed a small page file on my HDD to handle the case. It increases stability.

Edit:
Seeing that you're planning to use AUTOCAD, you may want to think of investing in a bit more RAM but definitely keep a pagefile around. RAM is cheap and you'll get far better performance keeping all of that info in RAM rather than needing your pagefile.

Solution 2:

According to Microsofts MSDN blog, put it on the SSD to get the best performance.

Should the pagefile be placed on SSDs?

Yes. Most pagefile operations are small random reads or larger sequential writes, both of which are types of operations that SSDs handle well.

Solution 3:

I have been told that using an USB Memory Stick (USB 3.0 and of significant read/write performance) is the way to go. When the memory stick gets beat up or fails, just replace it. Saves the SSD. Disclaimer: I have not done this but will be going down this path real soon.