Disclaimer: I own an SKS and an M14 (similar to mini14) and I used to own a Mini 14.
It comes down to what you want it for. You mentioned this is for Self Defense/SHTF. I do not have access in Canada to the extended mags for the SKS, but if you still need to load it with stripper clips, take it off your list. I have seen the detachable mag mods for the SKS and from what I understand they are hit and miss, but I could be wrong.
The reason I do not like the SKS is the stripper clip reload. Does it work? Yes. Can I mag change twice as fast with a detachable magazine? Yes. Not to mention you get 20/30 shots with the mini-14.
Now, the Mini-14 was not meant for extended shoots. They used to have a paper thin barrel (They did when I owned one, not sure if they still do) that will over heat very quickly. The mini 14 is more accurate than the SKS but still not "accurate" compared to an M16 (in my experience. YMMV).
When the shooting starts, you go through ammo fast (unless you are WELL trained). This leads me to want a higher mag capacity. If I had to chose on your choices, I would chose the Mini 14 based on mag capacity, speed of reload and moderate reliability (with good mags). I am not commenting on the Kel-Tec because I know nothing about it, but civilian target market firearms are generally less reliable than ones designed for the military.
I saw a couple of AK's on Armslist for under $600, so they could be a good option also (better than both the above IMHO). As for the wife ok'ing the tactical rifle, that is something only you can answer. My wife was not big on it, but when we had the available money (no debt, a little extra cash) I told her I was going to spring for it. That said, I had told her for years (10+), that when the time was right, I was going to buy one. Not to sound like an ass, but when it comes to family safety, I have a couple of things I will not negotiate much on. For the longest time, I had an 870 so I could take care of things around the house, so Tac rifle was not a top priority.