It's simply that the K-Var is a real duty stock that you can depend on while the tapco is a cheap alternative for someone who doesn't need or desire the durability of a real duty weapon. That said, even the K-Var is not as beefy as I would like to see on a stock for an AK, but it does the job and I would feel confident in using it to bash something or someone if I had to.
Anyway, probably the biggest/most noticeable difference is in the joint. The K-Var has a strong, rigid joint that fits very firmly into the receiver (I had to hammer mine in). The tapco joint just isn't as good. The K-Var joint fits so snugly that it will hold without screws (but of course you should secure it with screws to be safe).
This is not my picture, but here is a comparison, tapco on top, kvar on bottom.
Next order of business is Warsaw length or NATO length. Nato is longer. Warsaw is tiny. If you plan to shoot the rifle American target shooter style, with the rifle held across your body, get the NATO unless you have tiny arms. If you plan to shoot tactical style with the rifle held straight out in front of you, or if you just want more authenticity, get the Warsaw length. I went with Warsaw. Yes it is tiny but that makes it very handy for tactical style shooting.
It's all up to you, get what fits your budget, needs and desires. Just don't put any M4 crap on your AK!