The iPhone 6s and 6s Plus went on sale in the wee hours this morning (just after midnight Cupertino time) and demand is already making a pretty large dent in availability.

If you order on-line for delivery, the waiting time for some phones is already at 2-3 weeks (for delivery between September 26th and October 3rd).

The in-store pickup program is for September 25th only. If you go that route, note that the 128 GB iPhone 6s Plus is already unavailable in many markets, no matter the color or carrier. In my area, the 128GB 6s is only available in a few stores, and in a single color.

If you do go with in-store pickup, consider joining Apple’s iPhone Upgrade Program.

In a nutshell, you agree to pay a monthly fee, to Apple (actually to Apple’s bank partner, Citizens Bank, N.A., doing business as Citizens One), for a minimum of 24 months. The fee is based on the phone you choose. As an example, if you buy a 128 GB iPhone 6s Plus, you’ll pay $44.91 a month. If you bought that phone outright, you’d pay $949, plus $129 for Apple Care+.

Some math:

• Upgrade Program price: $44.91 x 24 months = $1,077.84
• Pay Up Front price: $949 + $129 = $1,078

As you can see, these prices are pretty much the same. But the Upgrade Program has a terrific side benefit. Let’s say Apple comes out with a new phone in a year (not a lock, but a reasonably safe bet). After 12 months, you are free to switch to any other phone that Apple sells. You trade in your phone (straight trade, no money changes hands) and get your new phone, start a new Upgrade Program agreement at the price associated with that new phone.

With the Pay Up Front program, you keep your old phone, which you can pass along to someone else, or trade in or sell when you are ready to buy a new phone.

With the Upgrade Program, you don’t get any money for your trade-in, but if you trade in after a year, you no longer continue to pay for that old phone. In our example, with a trade-in after one year:

• Upgrade Program price: $44.91 x 12 months = $538.92

If you swap phones every year, this is definitely worth considering. When you do your own math, don’t minimize the value of your used phone. After all, Apple will no doubt take that same phone and refurbish/resell it. Selling a used iPhone is relatively easy to do.

Also, consider how important AppleCare is to you. If you are very careful with phones and willing to chance going without insurance, that should factor into your decision. With the Upgrade Program, AppleCare is included and is not optional.

UPDATE: Here’s a link to the legal agreement, which provides a few more details. [H/T Jarrod David]