First of all: Hail to thee young padawan, for honouring the ancient ways!
The outputs are looking different because two different typesetters have been used (which used different fonts on top of that).
When using the -Tpdf option, the output is rendered for a PostScript-like typesetter(*) and the standard fonts (Times-Roman, Times-Italic, Symbol) are being ...
I think LaTeX is more of a markup language that automatically outputs the text based on style selections, while troff allows more precision but has less automatic style support. You can use both for either task, but I think each has its favored use. I use groff when I need precise text positioning.