Didn't Lanse recently make a video comparing 7018 and 6011 welds done both AC and DC? If I remember correctly (never a given, nowadays), the 7018 welds on AC turned out to be slightly stronger than those done with DC.
7018 is not a purely DC rod like 6010. Lincoln lists DC+ as the preferred polarity for their E7018 H4R (Jetweld LH-70), but also lists the setting range for AC for all sizes up to 3/16" rods. Above 3/16", AC is listed as the preferred polarity.
I've never found a 7018AC that I like. I haven't been forced to use it much, but the few times I've tried it, it's shown a tendency toward erratic behavior, and occasional porosity. I'm not saying it's bad---just that it has never worked well for me. If I only had an AC welder to use, I might still use regular 7018 (run it a little hotter and try real hard), or take the time to shop around until I found a 7018AC that I liked. But I've already tried at least five brands---Lincoln, Weld-It (Hobart's second best), Forney, and two or three mystery rods which weren't any of those---and haven't been impressed with any of them.