Both have their strengths & weaknesses in my eyes, though it ultimately depends on which you personally prefer I suppose.
When Kane debuted in 1997, he was a great monster who plowed through everybody. The problem with being a monster, however, is you have to eventually go through some changes. Being a human juggernaut is great for a while but it did get stale after about a year or so. Kane had an aura of intimidation in 1997 that he doesn't have today and part of that has to do with the fact that he was brand new, someone we'd never really seen before and was given a deep storyline that connection him in a familial way to Taker. He also had someone to talk for him, namely Paul Bearer. However, given that they were "brothers" and both these "supernatural" beings, it was just a matter of time before Kane got labeled as poor man's Undertaker. And, eventually, that's ultimately what happened. Over the years, Kane has made little tweaks to his character here and there while ultimately keeping the core essence going. While Kane hasn't reached the heights of Taker, he's someone that's been booked strong frequently throughout his time in WWE. Personally, I think Kane is underrated for his promo and in-ring ability and has been a constant threat in the upper mid-card to main event scenes. That's not to say that he hasn't been involved in some crap feuds. I can't think of a major star in wrestling during my lifetime who hasn't been involved in some crappy stuff now and again.
With Kane's character as it is now, mostly playing the straight man in an unconventional partnership/friendship with Daniel Bryan, it's shown a different side of Kane. I don't think anybody really expected Kane to click with Daniel Bryan but he did and did so in a great way. While I think he will eventually go back to being more of the monster he's generally been, Kane has shown himself to be versatile. He can be more than a one dimensional monster that rampages about and destroys whatever's in his path. Given how long Kane has been around in WWE, this was a welcome change in my opinion. Kane, while still in great shape, is in his mid 40s and wrestlers in his age range can often have difficulty staying relevant.
If WWE brought Kane in today as they did 15 years ago, I don't think fans would have been as into it as they have been. There are far too pessimistic fans today who aren't willing to suspend disbelief so that they can buy into Kane being this "supernatural" force. Same thing with Taker. They're both still over because they've earned fans' respect over the years. When Albert returned as Tensai, he was a one dimensional monster who wrecked things that the fans simply didn't accept. Kane was essentially the same thing in 1997, granted he had a manager/handler/mouthpiece, but he was basically just a big guy that wreaked havoc. I think a Kane like character today would ultimately be given similar treatment as Tensai, maybe even more because of the "supernatural" stuff.