I see this as two questions in one. The first is about the fingerprint scanner, and the second is about buying now or waiting for a new iPad.
Regarding the fingerprint scanner, at this point in time it is used only for unlocking the device and for getting around password prompts on the App Store or iTunes Music Store. Any time you reset the device you'll need to re-enter the complete password to unlock the device and also to make a purchase; once the password has been entered, you'll be able to use the fingerprint scanner instead. If you reboot frequently and/or don't make purchases frequently then you may find that you're entering the full password for the store fairly often.
Most people seem to think that the fingerprint scanner is useful on their iPhones, but don't see the utility on their iPads. The thinking is that the average user may use their phone in short bursts many times during the day, but will use their iPad for long stretches at a time (and thus, the time and effort saved by a fingerprint scanner would be minuscule). This one is up to you. I use my iPad for work, where I lock and unlock the device multiple times per hour. A fingerprint scanner in an iPad would be a wonderful feature for me. If you unlock your iPad three times per day, the fingerprint scanner might not have as much utility to you.
"Buy now or wait" depends heavily on your uses and on you as a person. Is the iPad going to be used for work, or will it be something very useful to you? Do you have any qualms with selling your devices (whether on services like Gazelle or through places like eBay or Amazon)? If you answered yes and no, respectively, then you might as well buy now, use the device, and then take a small hit in upgrading with the next iPad Mini is released. We're all anticipating that the next iPads will have TouchID, but there's no guarantee that they will. It's also possible that the iPad Air 2 will have TouchID, but the iPad Mini will not. Depending on your anticipated usage, five months may be a lot of time to wait for a feature that might not show up.