What are the major differences in these 2 programs. What would motion be able to do that AE can't and the other way around.

Not that I have used After Effects, but I do use Motion. Motion is limited primarily limited to 2D work, whereas AE can do both 2D & 3D. Motion is also designed to run in realtime (depending on your system) and work with short clips.

For what its worth, I believe Motion is an incredibly useful tool considering its low price--plus I dont need to do 3D work.

I have used both and the 3D is a major difference. Beyound that I'd say that for many simple tasks (esp. titling) Motion will allow you to get to a nice finished product faster if you are not already accomplished with AE. Motion will do some effects that AE won't or that are much harder to do quickly on AE.

i use both and i'm an ae head, use it every day, i don't see motion replacing ae, but definatly complimenting it, some features overlap

both can do particle effects, but its so much easier and faster in motion...

for compositing i like ae better, i like masking better in ae, for titling i love motion, nothing even comes close in my eyes...

although you can achieve the same results the paths are very different, motion uses behaviors, which allow you to do away with tedious task of keyframing on your everyday animations, or bug creations, or whatever you do day in day out

i also like the rt preview ability of motion, with that said i have invested years in ae, built workflows for myself and spent hundereds if not thousands on plugins and updates..i won't be leaving ae anytime soon...

By far the biggest differences to me have already been mentioned. Those are:

2D vs. 3D
Realtime Preview vs. Ram Preview

I'm actually torn between those two right now as well.
But, like Robert, I've been using AE for years... I have my workflows, I have my plugins, so it makes it hard to change.

When it comes to Motion, keep in mind we're talking aobut a first generation app, and comparing it to a sixth generation app (AE).

Mac is said to have a new version of Motion to show off at NAB, no telling if it's a major upgrade or a tweak.
Either way, I don't think motion is going anywhere anytime soon, and of course, neither is AE.