Here's what I like about both of them:
I tip my hat to Steve Whitmire because he had the huge responsibility of taking over after Jim. Steve's singing voice is outstanding, and his Kermit sounds much cheerier. He really does the voice acting well, too. Jim didn't have to alter his voice much when performing Kermit, but Steve does, and he is great as the current performer of KTF.
Jim's Kermit was more versatile, capable of moving viewers to tears ("Bein' Green" or the scene in The Muppet Movie 1979 where Kermit chats with himself under the stars) and making us laugh out loud (routines with Miss Piggy, Grover, Fozzie, Cookie Monster, Floyd, Scooter, & others). Jim's Kermit had the ability to create memorable scenes with any character he interacted with, especially because Jim had a great rapport with the other Muppeteers.
So by a slight margin, I prefer Jim Henson's Kermit, even though I was born a few years after Jim's death.