To me, it's really a personal choice.......what is the price difference between the 2 options.....if it's all about saving the most money, then tethering may be cheaper......a wifi iPad is certainly cheaper......Will you have access to LTE where you are......And how do you like to work?
I, myself always buy products that stand alone as I don't want to have to set up 2 devices to work on one device.....and sometimes you just want to be able to do it quickly, which I guess you could use your iPhone in these instances but I don't work like that myself, the iPhone screen is just too small for me. I want to be able to just open my iPad and be able to get what I need straight away.
Do you travel at all? Do you want to be able to use your iPad overseas?
There's also GPS to consider, I use my iPad for this reason and often.....the size of the screen makes the iPad a wonderful tool for navigation and I don't even need a charged sim.....the chip is built into the iPad.....this feature alone makes the cellular version worth it for me.
There's also the point that your iPhone will drain battery quicker when it is tethered although I do notice my Air drains battery quicker when I do use cellular.
Then, there's apps to consider....will you have access to wifi to download apps??.... as large apps cannot be downloaded on cellular.
Sorry, if I haven't really helped you but it's all food for thought!
Good luck with your choice, but let us know what you do finally buy!