They are both accumulation punchers, witn probably similar power but different styles. As far as one punch power is concerned, Shavers and Lewis are far superior to Foreman and Wladimir. With young Stevenson as an obvious number one in this department.
Based on a single, straight right hand I'd say the power is roughly equal. Wlad has a much more powerful left hook & jab though.
I think one thing which makes Wlad's KOs come later and causes them to be somewhat less spectacular than Foreman's and Lewis' is his new style to protect his chin. He doesn't commit to hooks and throw them in a wide open fashion (a very powerful punch) like Lewis and Foreman did, because he is worried about protecting his chin. Mechanically, the straight right hand he depends on is less powerful than a looping hook thrown with the whole body. He throws short hooks or hooked jabs, but not looping totally committed hooks, which reduces his likelihood of early/spectacular KOs. A thing about Foreman and his power is that the man has always been very naturally DENSE. Wlad is a lanky guy who lifts a lot of weights and added his muscle, but Foreman has a lot of thick natural muscle. Thick legs, a thick trunk, back, and shoulders. Foreman has a lot of natural power in his body, not power added in the gym. I think Wlad also has natural power, but not as much as Foreman. I think Wlad is close to Lewis in terms of power, but watch a KO highlight video of Lewis and compare it to the recent Wlad. Lewis throws with wild athletic abandon, fully committing his punches with his whole body. Wlad fights much safer and doesn't commit his whole body. He throws robotically. Lewis also might have been a bit more accurate than Wlad.