I'm also looking to upgrade from iPad 2. I'd go with the iPad Prosumer as it's the most future proof with beefiest specs but wait till it goes on sale since it shouldn't be more than $600 to $650 max for the base model.
iPad Air 2 is still laggy for some cases and 2GB DRAM will be a bottleneck for heavier split view productivity apps and the reason Apple equipped the iPad Prosumer with 4GB DRAM.
iPad Mini 4 has a slower A8 so it'll be worse than the iPad Air 2 A8X.