My HTC One has a 4.7" screen, and I'm trying to decide if I need a Mini in addition to the phone and the Air I already own. The Air is much more portable than my iPad 4, and I do carry it outside. The iPad 4 was taken outdoors about 4 times.
If you want something that'll fit in a purse, the Mini is better. If you have a shoulder bag, the Air would be my choice. Carrying in bare hands is easy enough with either device. The Air's screen will offer the more dramatic difference in terms of size, so I'd say get the Air.