Is it pointless to have data on your iPhone and iPad at the same time?

Would it be completely pointless to have a data plan on both your iPhone and your iPad?

A little background information:

My T-Mobile contract expires soon, I plan on switching to Verizon. I haven't upgraded my phone since 2008, with this said, I would also purchase the iPhone 4S device under contract for $100. I'm also getting an iPad 4th gen. For Christmas.

My question is, wouldn't it be rather pointless for me to get a data plan on both my iPhone and my iPad? The only time I would really use wireless internet in public is on my train ride commute - which would be more appropriate to use my iPad to browse the web, go on YouTube, watch movies, or even listen to music.

The only problem is that using internet on my iPad would essentially be less portable than a phone. Meaning, I couldn't go on the internet as many places as I could if I had internet on my phone.

My question is: would I be able to tether internet from my iPad to my iPhone and vice versa? For example, I got a data plan on my iPad, could I tether the internet to my iPhone and use the internet on it?

or

I got a data plan on my iPhone, could I tether the internet from my phone to my iPad and use internet on my iPad? (with a jailbroken iPhone 4S)?

Added (1). Okay, thanks for the advice, guys. As a follow up question - does Verizon offer the wireless hotspot option? Or is that limited to AT&T?

I think you already can use iPhone as a portable hotspot

Get it on only lone device and use it as a portable hotsport for the other.

If you'll always have your phone with you when you will be using your iPad for Internet, then instead of paying for 2 data plans, you should pay for 1 data plan (for your phone), but also pay extra on that plan to enable wireless hotspot.

That has two advantages: first, it's probably cheaper than 2 data plans, and second, it means that you can connect anything that does Wi-Fi to your phone's hotspot, not just your own iPad.

My friend has an internet data plan for her iPhone 4S, and she was able to share her connection to my iPod when we were having coffee the other week. It was pretty fast too even if we were both sharing the same connection. If your phone has a mobile internet data plan, then you wouldn't have to spend double for another plan. You just need to have the phone with you to get the iPad connected. You might need a higher internet data plan, though, if you will be consuming more amounts of data. Some carriers might also charge you for sharing the service. It would be best to check your service provider first. If you want to know how to share your iPhone, here's a video