The easiest choice would be Graham at this point because so much of having a good Kicker is not just his Leg Power/Accuracy its his psyche in big moments...and that's something Demke hasn't put on display yet.
While Graham has dating back to last season.
If I were coach while I was resting Graham I'd hope that a game winning situation came up in one of these preseason games and see how he fairs with a game on his shoulders. Only then would I be willing to move on from Graham completely.
We must remember that thought Demke and Graham have up to this point missed 2FG's a piece I believe, Demke's miss notably came in a game winning practice situation(will posts link later if I can find article)
So yeah while I'd prefer the younger leg of Demke's (nice to he his kicks go right to the middle rather than hooking absurdly) at this point Grahams psyche and Demke being a rather unknown in similar situations Graham gives the upper hand IMHO