Paying For PCalc
The Mac App Store route is easier - if you have an existing iTunes account, it's pretty much a one-click purchase. The application will download and install automatically and the store will keep you up-to-date when we have any new versions out. There are no serial numbers to keep track of or anything like that.
On the other hand, if you purchase via Kagi, we get a little bit more of the money (Apple takes 30%, Kagi around 10%) and we get to know who you are. After you order, Kagi will email you a serial number, and you can download your own copy of the application here.
Kagi also will sell you discounted five and ten packs, and a bundle with our other app DragThing. They also support a much wider range of purchasing options beyond credit cards.
So really, it's up to you!
Thanks for supporting indie developers in general, and our company in particular.