It's important to test across various screen sizes, which the iOS simulator is good for, but it's also important to test on real devices where possible. I currently have an iPhone 11 Pro, an iPhone 6, and 2 iPod touches. Out of all these I find the iPod touch to be the best device for a lot of iOS development.
Cost
Especially when starting out developing any extra cost can be a burden. The often-recommended iPhone SE starts at $399/£399 but the iPod touch starts at $199/£199 which makes it a comparatively great deal.
For the same price as an iPhone SE you can have an iPod touch kept on the latest developer beta and another on an older version, giving you a lot more OS version coverage.
Screen Size
The iPod touch has the same screen size as the zoomed iPhone SE, so if the design fits on an iPod touch you know it works for the smallest screen size available. Testing on larger screens is also important but this can be done in the simulator, or your own personal iPhone.
Processor
The iPod touch has an A10 processor, while the iPhone SE has an A13 processor. There are iPhones and iPads that still receive iOS updates and have slower processors, although none of these are currently sold by Apple.
It's not perfect but it's going to be easier to find performance issues with an iPod touch than any other iOS or iPadOS devices currently sold by Apple.
Colours
The iPod touch comes in 6 different colours, the iPhone SE comes in 3 colours. 6 > 3.
Lack of Hardware Features
The biggest downside to the iPod touch is a lack of hardware features. It only comes with WiFi, a front camera, a back camera, bluetooth, a three-axis gyro, and an accelerometer. This means it's missing:
- Any form of GPS (although location services are still available)
- A compass
- A proximity sensor
- A barometer
- A LiDAR scanner
- An ambient light sensor
- A magnetometer
- Any form of biometric authentication
For a lot of apps this is unlikely to pose a problem but if your app relies on one of these it may make the iPod touch a bad choice for app development.
A Developer iOS Device
Ideally I would like to see an iOS device or iOS version specifically aimed at developers that could emulate a lot of these things, e.g. slowing down the processor, disabling hardware, reducing the resolution of the screen, install older iOS versions.
Apple offer the security research device so it may be a possibility in the future, but for now I'm going to stick buying iPod touches.