Unless you have a lot of i/o I wouldn't stress about it too much. I have WD Green drives in my unRaid build and I still get 30MB writes despite them being 5400RPM drives. There are a lot of factors that come into the disk performance such as total files, block size, etc.
They aren't disagreeing with you, but since the mini uses 2.5" drives, a 5400rpm drive is cheaper than a 7200rpm drive. There will be a speed difference, but it all depends on what you're doing to be using it for.
My suggestion would be to get the Fusion drive, it will cost a little more but will be worth it. Early tests are showing around 400mb/s read/write speeds.