If you leave Personal Hotspot and Bluetooth enabled on the iPhone and tether the iPad via Bluetooth, then it's easier (and uses less power to boot).
It's also slower, a lot slower. You'll never take full advantage of even the slowest LTE connection over a bluetooth tether, and the iPad needs the faster connection more than the phone does. And I would disagree about the battery usage being less. The battery drain is still pretty bad.
I used to go Wifi only on the iPad, but with the Air I went cellular and got a separate plan for it. Couple reasons:
1. Free data from T-Mobile (200MB/month)
2. That battery drain on the phone, and the heat generated.
3. It's a lot more convenient. Instead of setting up and breaking down hotspot connection for my iPad, it's just there and it just works.
4. The iPad having its own cellular connection makes sure that the Find My.. app will work if I lose it, instead of hoping and praying that it finds a WiFi connection.
Not only that, but honestly, the iPad has the bigger battery, and in reality that makes it better at Mobile Hotspot than the iPhone is.
As an aside: T-Mobile really knows what they're doing with that free 200MB/month deal. After a few months of having it on my iPad, I saw the coverage was decent enough for me to give it a shot on my phone, and now I ported my iPhone AT&T number over. If it works out, I'm saving a lot of cash, even paying for data plans on both my iPhone and iPad. If it sucks, well, I'll wait for T-Mobile to pay off my ETF, then switch back to AT&T contract-free, and still save a little bit on their plans.