Hmm, you could change to MiFi for both devices, yes. As far as upgrading to the 10GB/mo plan - I would advise you look at your total usage for both desktop and netbook combined. If you don't use your 5GB as it is, it may be suitable just to stay where you are now.
Speeds - not sure, though I don't believe it would effect them TOO much. If you're downloading from one, and working on the other, I'm sure you'll notice some adverse effects. Otherwise - honestly not sure, someone else would have to answer that for you. You could try browsing on the site a bit, you should eventually get an invite to chat with a sales rep and they may be able to give you more info on the speeds.
As for the iPhone: Yes, it can be tethered - however, it's an addtional $20/mo for 2GB of bandwidth. That is in addition to the $29.99/mo for the unl. data package (required for smartphones). So, if you use more than 2GB bandwidth total each month, probably not what you need.
But, if you're looking to just get the iPhone - sure, the data package isn't cheap, but the iPhone is a good phone with alot of features, and I personally think it's easier to fit in your pocket than a netbook
The MiFi should be the same speed as your current Netbook because it's on the same 3G network. So, you should be able to decide how that compares to your current AT&T speed for your desktop. I suspect it will be significantly slower. It's also impossible for us to estimate how much data you will need because we don't know how much data you are using on your desktop. It could be a few GB or it could be 20 GB?? Also, how is your 3G signal reception where your desktop is located? Have you put your netbook at that location to test the 3G reception? It may be fine or it may be terrible. You are going to have to make a trade-off of portability (netbook and Mifi) versus speed (AT&T and desktop). Which is more important? It's a difficult decision for sure.