1. The chance, weighted by amount, of a price hike/salary increase
2. The chance, weighted by amount, of a price decline/salary decrease.
As inflation is generally an upward trend, clearly no. 1 is more likely and thus the best bet.
Also someone may have some assets they could turn into liquid cash if I really needed to but may not have a lot of liquid cash on hand, so there would be little risk in paying something in installments as long as their assets would be enough to cover it in a pinch.
I am not sure it actually applies to the cost of a phone. Chances are, if those matter, you'd be better off with a cheaper phone to begin with. (Substitute phone with other luxury articles).