I just bought an i7 256gb...and I'm getting the fan on full blast and heat with lightweight games from the app store...I've installed Lightroom, but haven't yet copied over my catalogs and worked in it yet. So I'll see how that goes. I'm expecting fan noise during import/preview rendering and then maybe it'll settle down for editing as that's more start/stop on the cpu.
It's tough researching this online, because there's not a lot of "specifics" to help me determine if what I'm experiencing is a hardware issue, bad programming of the app, or just flat out the way it is and to be expected...
Regarding the OP, I would say go with the i5. And from all indications online, you will only get heat and fan noise with sustained loads. Usage being gaming, video rendering/transcoding, rendering 3d models, things like that. What I can't figure out for the life of me is "what level of gaming" to expect the surface line to run without the fans having to go nuts....with my i7....it seems to be practically any game whatsoever...
Also of note, even with the i7, streaming HD video via the Netflix app resulted in just a whisper quiet fan and the device remained cool (for me).