If you are an iPhone user, it makes more sense to buy an iPad than it does to buy a cheap Android tablet.
By sticking within the Apple set of devices, you only need to buy apps or in app purchases once and you get them all across your devices. This also simplifies watching movies you buy from Apple, as well as music if you buy music through iTunes.
Sticking with the operating system that you know is one of the biggest reasons to match your phone to your tablet. I tried having my dad use an iPad mini while he used an Android phone and there were too many things that got more complex by mixing these devices. The same is true for using an iPhone and an Android tablet.
You can even copy an item on your iPhone and paste it on your iPad or AirDrop files form one device to the other.
Your computer may not matter as much, but going from phone to tablet, stick with the same system.