My wife had the same question and was using an sp3 I7. She wanted the new sp4 because of the improved pen and screen, so she preordered another sp4 i7. in the meantime, she traded her sp3 for an m3 sp4 to try out the fanless version and see what it's performance was like.The question she had was, mainly due to the fan, should she go i7 or m3? She uses lightroom and photoshop, along with the other task. once she finally got her spr i7, she found that there wasn't a significant enough improvement for what she does to justify the additional expense or noise of the fan.
While you're looking at these, I would highly recommend looking at HP's Spectre x2. it's available with a more m processor choices(m3, m5, and m7), less expensive, and with an LTE radio for Verizon. the problem is that it's got a smaller screen, and it simply isn't a surface. I've become slightly leary of this after owning devices where they cut support for updating drivers, although I have no reason to suspect HP doing this.
Sorry just to clarify are you saying your wife prefers the M3 and it's good enough for her Photoshop and lightroom usage?