Upload & Sell: Off
There's no option with dual SSDs. It's either 1 HDD, 1SSD or 1HDD+1SSD. The decision really depends on what else you're doing with the computer. What you're actually doing with the computer (in terms of large panos, etc) won't matter a whole lot when it comes to your decision. SSDs can make everyday use a lot nicer, so it's hard to recommend against them, but the upgrade price from Apple is pretty outrageous. Unfortunately doing that upgrade after-market isn't as simple as it should be. Also take into consideration how you'll be connecting the ext. photo drive. If you have big raw files, you may see a difference between Firewire and Thunderbolt, so spending the extra $500 there may make more sense. Or you could go with the internal SSD and do your editing there, and then move to the ext. for archiving, where speed isn't as important. If you need extra space for music/movies/crap, adding another external 1TB drive later is really cheap. If you think 256gb won't be enough for even your "core" system (operating system, the applications you use, etc), you may end up doing a lot of juggling of data and lose out on the speed improvements.
If it's in the budget, I'd say the 1TB+SSD is a safe bet.
FWIW, "occasional offsite back" makes me cringe. Continuous cloud backup is so cheap now, if you can spend this kind of money on a computer, there's no reason not to have it.