Lumia 710 vs. HTC Sensation vs. iPhone 4

Well, I've decided to sell or trade my BlackBerry 9860. Mostly because BBM isn't working because of an issue with my carrier and phone, but also because the on screen keyboard is one of the worst I've ever used. I'm in a bit of a quandary, however. I have virtually no money, so I'm limited to what I can sell the phone or straight up trade for.

I have 3 offers so far. 300 bucks, which I would probably use to get a Lumia 710 (They're only 255 brand new off contract, which is just ridiculous in a good way). An HTC Sensation, which I've been interested in after seeing its getting ICS this month (well late March, but whatever) or an iPhone 4. Each has their own strengths, yet also glaring weaknesses. All of them do appeal to me, because I've also had a lot of useless offers that I won't even bring up.

I'm really torn on which to get, and I just can't make up my mind.

What do you think. Which would you choose, and why? Any other suggestions in the price range?

Any advice is appreciated, thanks in advance!