If you can find the old Thermold magazine's for the Mini-14. They work great. I own 9 of them and never had an issue one with any of them. Other then the old Thermold's for the Mini-14, factory is the way to go.
For the Mini-30, I just picked up 4-20 rd factory magazines. NICE! There is no movement at all to the magzine after its locked up in the mag well. Unlike the 30 rd magazine's I have (no clue who they are made by, been too long ago) which have a lot of back and forth movement locked in.
With aftermarket magazine's it's hit or miss. Some will work great while others turn into epic fail's. When you buy a magazine, make sure when you buy it, there is a good return policy just in case.