Mini boxes were used as loaner dev boxes because it was the easiest & cheapest way to provide an ARM box to developers. But there is no guarantee that Apple’s lowest-end, lowest priced, possibly lowest-margin machine will be updated to ARM in the first wave of new machines. (Apple similarly offered a transition kit in summer 2005 with an Intel Developer Transition System box in a PowerMac case that too was required to be returned to Apple).
When Apple announced the first Intel Macs in January 2006 they were a Macbook Pro and an iMac. A month later they offered the Mac mini with a low-end (and if I remember, vastly underpowered) Intel option. Six months later they completed the transition with new variants of all pre-existing PowerPC Macs.
Analyst Ming-Chi Kuo, who has leaked many accurate details over the years due to close ties to sources in Apple’s supply chain has reported that the first ARMacs to come out, at the end of this year, will be a 24" iMac and a 13.3" Macbook Pro.
So I wouldn’t count on being able to buy an ARM-based mini until maybe a year from now.
Should you wait? It’s up to you. But you probably shouldn’t expect to get a mini with an ARM processor this year.