my take on your situation:
1-Wait till June. You already have a EVO and get wimax going to and from work.
2-The Photon is a very good phone, it has a better radio than the EVO. I've gotten 4 bars with the Photon as oppose to 2 or 1 bar with the EVO in the same location at work. But if you are into rooting and Roms the EVO (or more to the point any HTC phone) is way better than a Motorola one.
Removable batteries, I feel your pain. With the EVO it was having a charger at both home and work having it plugged in, as well as in the car. But if walking out where there is no outlet available, I would have to have extra batteries on hand.
With the Photon, the same thing but not as much. I've read that the newest Samsungs and HTCs have better management and thus lasts longer than the previous models.
The EVO is EOL and word has it that the Photon will be also very soon (as mentioned by Eric above). Teh Photon has great accessories such as the lapdock and I love it, but its still in its infancy.
3-While the saying is "the grass is greener n the other side", most fail to forget that it takes alot of maintenance (and money) to keep it green. Bottom line you may be spending way more going to the other side.
As Robert and others has mentioned, your area will be getting LTE and by the time its up and running, both HTC and Samsung will be pushing out their newest phones. Personally I would wait it out. Sprint knows that alot of the EVO folks are up for an upgrade in the June/July timeframe and are going to push out quite a few phones by then, that will take advantage of LTE. They know very well that if they don't all those that jumped ship from other carriers for the EVO line will have no problem jumping back. They simply can't take that chance.
As for the Nexus, yes its a great phone and all, but its a benchmarked phone, and being as such, there will always be other phones that are more powerful, and at times have more stuff that make it yesterdays phone. (Only thing going for it is the pure Android OS (and thats debatable with Verizon) and ongoing on time system updates (also up to debate).
TS