Both great phones... I like that the photon has an hdmi output. I like that it has 16 gig built-in memory. It's a snappy phone. I like the EVO 3d as well. The screen looks better imo but at the same time, the photon screen is beautiful just not to my liking... EVO comes with an 8 gb sdcard. The sdcard is not a deal breaker and they are both great phones. It seems the EVO out performs the photon, which is debatable. However, I like that HTC supports its customers customizing their phones by providing the unlocked bootloaders, etc. There are pros and cons to each phone. Ultimately it was the HTC for me, for others it may be different. Depends on the owner...
It also seems that anyone with a photon is trying to trade it for an EVO. Originally I had a little buyers remorse because of having no kickstand and no hdmi out but after having the phone for three days, I don't regret my decision at all.
My recommendation? Ask around. Read reviews (sprint.com also has some good user reviews). Go into the store and hold them. Then make your decision. Good luck!