And don't worry about getting both of them.. For most people, Wii Fit is enough.
Unless you want ALL YOUR WORKOUTS to be on the Wii... in that case, go ahead and get both!
I also heard some pretty good user-reviews for the Gold's Gym version:
Amazon.com: Gold's Gym Cardio Workout: Nintendo Wii: Video Games
As long as you keep your leg movements from any fidgeting: It doesn't read your movements wrong.
And a cure for the whole "keeping at the same pace": Puppies.
When you chase after the puppies, you follow them instead... on longer distance courses the puppies can go pretty fast.
For some odd reason, at a certain speed: Your leg movement stays constant... small jogging movement usually results in inconsistent distance between knees, hips, and distance from feet and ground. But a faster jog usually results in almost perfect distance-consistency.
Blah blah blah: Anyways... also, if you unlock all the running modes, you HAVE to try my favorite one: Free Run.
In Free Run, you just put the Wiimote in your pocket (or hand), and you can change the channel! There's a voice coach in your Wiimote. Loud enough for you to hear through your pocket, quiet enough to hear your shows (of course, you can adjust the volume in your Wiimote). It'll tell you whether to go faster or slower, in order to keep a good pace.
So as long as this coach is on, you can watch TV... or be crazy like me: Play ANOTHER game on your TV while jogging!
And since you're not limited by seeing your Mii run, you won't feel compelled to run in place... often times in Free Run, I run in circles or back and forth.
I LOVE Free Run.
EDIT: Ah yes, and here's another thing...
Some people have said that as long as the Wii is in your room (no matter how large), you can Free Run anywhere within the room...
However, a rare few have claimed that they've jogged all around their house! Downstairs, upstairs, etc~! Those few have claimed it makes doing the laundry fun.
Not much of a way of confirming it without trying it... I haven't tried it yet, but I'll see if I can later.