I own an iPhone 4 and an iPad. I used the iPad almost all day every day until I got the iPhone 4. It turns out I use the iPad a lot less now but still far more than the netbook it replaced. I really like Nokia phones and you could do a lot worse than a Nokia phone and an iPad 2. Under normal circumstances, an iPad is a better investment than an iPhone because it doesn't carry a mandatory data plan with it. I find I use my iPhone a lot because I can read most documents and web sites in landscape mode on the retina display without any further zooming. The retina display is 3/4 the dimensions of the iPad display.
iPad 2 cost: $500. You couldn't blow money on a data plan even if you wanted to. Add $39 for the cover.
iPhone 4 cost: $199 plus $15 to $30 a month mandatory data plan for 2 years = $560 to $920 out of pocket. Also I would add a case. I don't like walking around with my iPhone 4 "naked" in my pocket as I don't want the front or back glass scratched so add $49 for a good case and invisible shield.
If you can afford the iPhone, it is a better "value" because you will more likely always have it with you. An iPhone 4 effectively replaces a digicam for family photos, etc. I only carry my digicam for very special occasions now. An iPhone 4 gaming performance won't be a good as iPad 2 but I noticed it was better than iPad 1. Then there is another thing to consider. A new iPhone will be coming out within a few months. If you need something now and can't wait, the iPad 2 is a better deal because it is brand new. If you can wait, a newer iPhone may work out better for you as it will probably match iPad 2 performance specs.
Of the 2, iPad is more of a "want" device and iPhone is more of a "need" device for most people but it really depends on your situation.