I'll skip out on the concept of how "professional" the apps in question are. Honestly, I just don't think that's the most relevant argument. Either package can certainly produce "professional" results - but this is all semantics.
But, I will add that, IMHO, AE is simply more mature than Motion. I'll give credit where it's due: Motion has come a
very long way for an app that's only at v.3. However, the devil's in the details, and there are things that you can do in AE that you can't do in Motion. But I can't think of anything that Motion can do, that AE can't. If they exist, I'd like to hear them.
The expressions in AE are very powerful! A couple of lines of script can create some intricate animations. The Puppet Tool in AE is really cool, and is only going to get better. If you're a Photoshop user, your "Layer Styles" can be imported straight into AE - or created from scratch in AE, w/o Photoshop, if need be. (Basically since we're talking about 2 Adobe products, the integration between Photoshop & AE is way more advanced than the integration between Photoshop & Motion. And likely always will be.) These are just a few points: this is, by no means, an exhaustive list.
Too, there are more plug-ins available for AE. Although the gap seems to be closing (due in part to the FXPlug technology) there are some cool plug-ins that AE has access to that Motion does not.
Everyone seems to think AE is
so hard to learn. I don't really get that, personally. Sure, it can be a little daunting in the beginning. But how can something that can create such complex results
not have, at least, some level of a learning curve. But if you read the
Meyer's book and/or do all the
AE CS3 training at Lynda.com, you'll find yourself quite savvy! Oh...and folks, don't be so afraid of keyframes. They are your friend!
Just my $.02 worth....