In terms of form factor, it's entirely user preference. The Mini or Mini Retina is aimed more towards the Android-sized tablets with the 7.9 inch display. The "original" iPads are larger, with a 9.7 inch display (these measurements are only the screen, not the entirety of the device, which is slightly larger). If you prefer a tablet that you can grip with one hand, the Mini is for you. If you don't mind a larger iPad and think that the larger screen size is worth the trade-off, the full sized iPads are for you. I wouldn't say one is better than the other, but there are trade-offs of each that you may want to consider.
Differentiating them individually within form factor (assuming we're talking retail), the iPad Mini original packs an A5 processor (like that of the iPhone 4S and iPad 2), which is the first dual core processor introduced for the Apple line. The resolution on the iPad Mini is 1024 x 768, like that of the iPad 2 as well. Overall, despite the lifespan of the A5 processor since release, it's still a very solid build and will see another few years of support before it becomes a legacy device. The iPad Mini Retina, packs the A7/M7 coprocessor, which offers 64 bit architecture, which is one of the first Apple devices to have it. Resolution is doubled compared to the Mini 1, and doubles the RAM, GPU and packs a bigger punch with the processor. If you're going for the Mini, it's recommended that you get the Mini Retina to get the most of your investment as it does have a noticeable difference in performance and graphics compared to the Mini 1.
With respect to the full size iPad, you have the option of the iPad 4 and iPad Air. The iPad 2 can still be purchased in some locations and authorized sellers, but it's essentially the same as the iPad Mini Original, just with a bigger screen size. The iPad 4 and iPad Air both have a 2048 x 1536 resolution, 9.7 inch display. The iPad 4 is using the 32 bit Apple Swift processor, whereas the iPad Air is using the 64 bit Cyclone processor with M7 co-processor (like the iPad Mini 2). The iPad Air is also slightly smaller in overall size than the iPad 4, and there is a bit of a noticeable difference between performance on the iPad 4 and iPad Air under graphic intensive scenarios.
The general consensus would be to get the newer device so that you get the most out of its longevity and performance. The only difference between which you get would be the size of the device. The iPad Mini Retina is essentially a smaller version of the iPad Air if you want to think of it that way, and with a slightly smaller price tag
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