WYeast 1187 Ringwood Ale

Did a stout, OG 1.049, been in primary for 14 days, oxygenated with pure oxygen. Had several days (2 or 3) a bit to cold 60 or so when I was gone for the weekend. The rest of the time it has been at 68-72. Checked the gravity today and it was 1.018 which is 60% attenuation should be 68-72% which would put it at 1.015 to 1.013. I know with this yeast I need a diacetyl rest. I'm figuring 75 or 76 degrees for that. I guess the question is should I give it a few more days at 70 to try and let it ferment out. Also is there any disadvantage to that and doing the diacetyl rest later?

I can't see why not. It's not like you're going to pass the "diacetyl rest window" in another week or two... unless you take it off the yeast cake too early. I would just leave it for 2 or 3 more weeks and then take another reading and taste it. I don't think extra time ever hurt a beer, especially in bulk.