What kind of devaluation to expect on iPhone SE?

I've been thinking about getting the iPhone SE. Even getting the 128GB version for another $100 is on my mind since I do need to carry around video presentations with me. But in trying to decide whether to bite off a $20 per month phone vs. A used iPhone 5 I'm having a hard time.

iPhone SE: $300 for 64GB, Wi-Fi calling, possibly $250 right now for a used/unlocked one (I have T-Mobile Simple Choice 2GB/month).

iPhone 5: $134 for 64GB used on eBay, no Wi-Fi Calling, even cheaper if I skimp out on memory. I'm afraid of either one losing iPhone and app support for the app I need. The app I need doesn't work on the iPhone 3G. Obviously if that ever happens the 5 is going to go first before the SE.

But If I can sell my SE for $80 to $100 in 4 years from now that would definitely make my decision. If I could get $120 to $150 in four years from now for a 128GB SE I'd go for that. But does this sound reasonable? Will it be worth that? Or should I get a 5?

Added (1). Simple Choice Plan is $100 plus fees and taxes for four lines. So I pay close to $30 per month for my phone. Adding a $20 per month is a chunk IMO.

I don't know