You can't steam through 3G. Its not powerful enough nor is it practical. Your plan will be used up in a few hours (if you get the 250mb) and a few days with the 3G plan. You'll probably also experience frequent buffering and long load times.
The 3G is merly for checking on your friends, sending messages, web browser, etc. Very simple uses for very simple featurs. You will not even be able to download games from the PSN (over 250mb) nor will you be able to connect to online multiplayer. (is it disabled and wont allow you to).
IMO, the 3G is not worth it.
Indeed, still happy i got the 3g version, just like to boast that i have an extra slot haha, but besides 3g, Netlfix on my Vita actually runs faster and better than my 360 slim/desktop/droid and laptop. Even when i fast forward it takes about 3 secs for it to catch up and no syncing or lagging problems at all, does take about a min to actually open the app, but otherwise its great to me.
3G not being fast enough for Netflix is a myth. I stream Netflix over 3G daily on my iPhone and have no issues with buffering or loading screens. The only reason I can see for the Vita to be limited to wifi using when Netflix is that it is a pre-paid data plan that just simply stops when you use all the data you paid for. The screen on an iPhone 4/4s is the same resolution as the screen on the Vita (the difference is the iPhone screen is 4:3 while the Vita is 16:9).
As you pointed out, on the 250MB plan, you'd blow through your data in 30-45 minutes. The 3GB plan would last you about 3 hours.
I am sorry but you are incorrect on a few aspects. It isn't a 3g thing, it is supposedly a data cap thing. It isn't a pre-paid scenario b/c AT&T is more than willing to charge you again and again for data usage (not only that but my iPad is prepaid and it works perfectly with netflix).
And on a side note how is it you think your iPhone is 4:3??? That would make your iPhone screen a cube. The easy way to picture this is an old tube style TV where the side is almost the same measurement as the top.
When I watch Netflix, widescreen HD shows are letterboxed horizontally. When I watch old style tv shows from the 90s, they utilize the full screen. My point is that horizontally, the iPhone is exactly the same as the Vita (960 pixels across). The difference is in the vertical (640 on iPhone, 544 on Vita). They both have the capability of streaming Netflix in a near-HD resolution over 3G. The iPhone does so perfectly, the Vita does not. I'm pretty sure the reason for that is to protect consumers from themselves.
The iPad plan, if you go over your allotted data, they just add on another GB for $10. On the Vita plan, apparently when you use all your allotted data, you're simply done for the rest of the month. Someone correct me if I am wrong.