Which method works better?
It depends on a few factors... in particular:
Type of knife? You can safely baton the spine of a folder (the spine of the blade, not the handle)--where all the impact will hit the blade, itself, but cannot safely use a folder lashed to baton, where the impact on the blade/handle juncture should instantly snap your knife blade right off the handle. A fixed blade gives you more options.
Size of knife? A knife as small as 3 inches or so can work acceptably when batonned. You need a bit more knife length, like four or five inches of blade, to make a reasonable pole-axe.
Kind of knife spine? A false-edged knife spine, or a double edged knife, is strongly sub-optimal for hitting with a baton. Too much force will go into cutting into the baton with the false edge, and not enough will go into cutting deeper into the targeted wood.
Kind of handle? For lashing a knife to a club for use as a pole-axe, you need a handle that is at least somewhat broad from the "spine" side to the "edge" side, and at least somewhat slender from the right side to the left side; you need this in order for your knife to not twist. You also need the handle material to be hard, like bare metal or micarta. If you are using a knife with a rounded or squarish handle covered with a rubber grip, then you'd probably best stick to batonning the spine of the knife.
Toughness of the knife? The impact generated by usage as a pole-arm is much greater than the impact generated by beating the spine with a baton. (On the other hand, the risk of accidental heavy lateral forces and twisting are higher with beating the spine with a baton.) I wouldn't use a stainless steel knife as a pole-axe. (Stainless steels tend to be more brittle than carbon steels--though, of course, a carbon steel blade still needs the right heat treatment to be as tough as possible.) Also, I would only try using a full-tang knife as a pole-axe.
How much chopping are you going to do? If you are just going to chop through one branch, then you'd be better off batoning, because you won't have to spend additional time finding/creating/modifying a suitable club to attach a branch to, then carefully lashing the knife onto it. Finding a decent pole, fashioning it suitably for your purposes, and lashing a knife to it can easily take 10-15 minutes. You can baton through one or two little brinches faster than that.
But when it comes to sheer chopping power... no contest; the pole-axe method makes for several times more powerful chopping than the baton-the-spine, at least. It will be much more powerful than any hatchet, too. Done right, it can even be considerably more powerful than a full-sized axe.
As for control: It takes a bit of getting used to, as does chopping with an axe... But once you are used to it, control and precision is not a significant issue. Certainly, if you were to compare the overall efficiency of:
(A) pole axe, with much more power but some control loss;
versus
(B) batonned knife, with better control but much less power;
the pole-axe would be far more efficient to use, overall.
By the way, here is a thread with tests of increased power with a pole-axe versus chopping with a hand-held knife (sorry, pole-axe versus baton was not part of the testing). The thread also gives details about how to lash a knife to a baton.
http://www.bladeforums.com/forums/showthread.php?t=209202
--Mike