between just those 2 options and you are not considering price i would say the iPad Mini. Also i going to assume that you are referring to Tab 3 8inch and that both devices are at the lowest configurable model.
As for why i would choose the mini I think it has a better physical design over the Tab 3 8 inch. i also don't like the samsung software that is usually added to their devices.
Now if you are open to any tabled i would strongly consider the Nexus 7 2013 model. i think that is the best android tablet on the market right now.
If you are looking for gdgt, then go for iPad mini
I would go for the Samsung tab 3 if you don't want to spend a couple of extra hundred dollars on storage space , I had both and I over the freedom that the samsung tab 3 had but when you using the iPad mini I was just not impressed besides the battery life there's nothing special .