I have the Photon and was thinking of replacing it when the GS2 came out. I played around with the GS2 at the Samsung store and was thoroughly impressed. The thing is that I was coming from an Epic and Samsung did a really poor job of updating that phone. And in the end, the signal was getting really bad at my office. The battery life was getting worse because of the signal problems.
I don't think you can go wrong with either phone. Both are really fast and incredible. Now that Google bought out Motorola, you can count on many updates and the Photon has had quite a few so far. I still think Samsung will be slow with the updates on the GS2. Also, the Photon is a world phone And that's pretty important to me as I travel overseas quite a bit. The GS2 isn't, at least not on Sprint.
My take is that the GS2 has better ROM support, brighter screen and maybe a tad snappier. The Photon has better battery life, is a world phone, better accessories and probably much better signal reception. Can't go wrong with either phone, if you ask me.