Keeping a smaller iPhone with Plus

Has anyone had any experience with keeping a smaller iPhone (4s or 5) as a backup for the Plus for use while running, cycling or at the gym? I've been leaning towards using a 6 as my sole phone since I run or cycle every day, but started thinking I may be better off with a smaller, cheaper phone for those activities anyway. The 6 and the Plus work, but neither are ideal.

My concern is swapping my SIM back and forth between the two phones on a daily basis. I've only swapped when upgrading each year, so I'm not sure if daily swapping poses any problems I should be aware of. Thanks for the help.

 

I run listening to Spotify and often a GPS app for tracking, and often on trails where I need the phone for emergencies, so the iPod doesn't work. For the gym it'd be fine, but I'm either on trails or the road most days.

And from what I understand, the Watch will require the iPhone for GPS tracking.

 

I ran with our Plus yesterday (or Friday), and my 6 today. But after pulling my damp 6 out of a water-resistant SPIbelt this morning after 10 miles, I started thinking that running with a less expensive, smaller backup phone wouldn't be a bad idea.