What's the difference between <mt:Foo /> and <$mt:Foo$>

Absolutely nothing. Traditionally, for only visual distinction container tags are usually input without the question marks (e.g. <mt:entries> ... </mt:entries>) and variable tags with (e.g. <$mt:blogname$>) but they are not required.

Other non required elements include:

• The colon between the mt and the rest of the tag name: <mt:blah> is the same as <mtblah>
• Proper casing (there isn't any): <$MTBlogName$> is exactly the same as <mt:blogname>
• The XML-style trailing slash on variable tags: <mt:EntryTitle />

Further reading

• What's New for Templating in MT4
• Template Tags in Movable Type

Thanks Allen, got it all cleared up! =)