I tried the mobile hotspot with my iPhone before getting my rMini. On paper it seemed like a good idea, though in practice it was not. My battery was sucked up quickly. I usually have the phone in my pocket and so while driving if my kids wanted to use the iPad I couldn't turn it on easily without getting into an accident, etc etc.
Try setting up a hot spot while on a bus with a bag on your lap, the Ipad in one hand and the the Iphone in the other. Also while Tethering has improved with IOS7 it's still unstable, if the Ipad goes to sleep sometimes you'll have to reset the phones hotspot setting just to get them to connect again.
rui no onna already posted about it, but I'll post it again and expand on it: tether by Bluetooth. The default tethering works by wifi. Wifi takes up a lot of battery on the phone's part. Perhaps partly due to that, Apple have programmed it so that the wifi hotspot shuts off if there is no transfer activity for 10-15 minutes, thus forcing you to manually reconnect afterward.
Bluetooth doesn't drain the battery nearly as quickly (especially with two devices that support Bluetooth 4.0 "Low Energy"), and the connection does not cut out due to inactivity. As long as you leave "hotspot" enabled on your iPhone and have Bluetooth enabled, making the connection after setting it up is simple, too. From one of the two devices, go to your Settings > Bluetooth and then tap on the saved device. Within about five seconds the connection will be made, and it won't break unless one of the two devices goes out of range or has Bluetooth disabled (which is my quick and easy way of breaking the tether).
I temporarily worked in an office with no wifi network, and tethered via Bluetooth for 6-9 hours each day. No problems, wonderful technology. If you've already bought an iPhone and you have a phone plan that supports tethering, it's the ideal way to "get your money's worth."
Granted, having cellular built into the iPad still represents the ultimate in convenience. Even though it takes me around 10-15 seconds to get the initial Bluetooth tether up and running, being able to pick up the iPad and use the internet immediately is obviously faster. Bluetooth represents a minimal drain, but it's still a drain on both iPad and iPhone that wouldn't occur if the devices weren't tethered. I don't think the extra cost of cellular in an iPad and the cost of a data plan for the iPad is worth it, but I can understand why some people would prefer such an arrangement.