For the first one remove the spaces. Whitespace matters for the set command.
set guifont=Monaco:h20
For the second one it should be (the h specifies the height)
set guifont=Monospace:h20
My recommendation for setting the font is to do (if your version supports it)
set guifont=*
This will pop up a menu that allows you to select the font. After selecting the font, type
set guifont?
To show what the current guifont is set to. After that copy that line into your vimrc or gvimrc. If there are spaces in the font add a \
to escape the space.
set guifont=Monospace\ 20