documentclass{article}
usepackage[T1]{fontenc}
usepackage{amsmath}
usepackage[skip=10pt]{parskip}
usepackage[table]{xcolor}
usepackage{rotating}
usepackage{array}
usepackage{booktabs}
usepackage{makecell}
usepackage[column=Q]{cellspace}    % changed to Q for simultaneous use of siunitx

newcommand{tableComment}[2]{
    begin{tabular}{m{4.5cm}}
        #1\[5pt]#2
    end{tabular}}

renewcommand{theadfont}{itshape}
renewcommand{theadgape}{}
renewcommand{theadalign}{cc}
setcellgapes{1pt}
renewcommand{cellrotangle}{90}

newcommand{TopRule}{Xhline{1pt}}
newcommand{MidRule}{Xhline{.5pt}}
newcommand{BottomRule}{Xhline{1pt}}


begin{document}

tableComment{1: Standard Lines, Head with texttt{makecell}, no additional spacing}
{
    rowcolors{1}{gray!20}{white}
    begin{tabular}{ll}hline
        rowcolor{gray!40}
        My Head & makecell{My second\ Head} \ hline
        some content & $dfrac{1}{2}$ \
        foo & bar \ hline
    end{tabular}
}
tableComment{2: Standard Lines, Head with texttt{makecell}, spacing with texttt{cellspace}}
{
    rowcolors{1}{gray!20}{white}
    begin{tabular}{QlQl}hline
        rowcolor{gray!40}
        My Head & makecell{My second\ Head} \ hline
        some content & $dfrac{1}{2}$ \
        foo & bar \ hline
    end{tabular}
}
tableComment{3: Lines with texttt{Xhline}, Head with texttt{makecell}, spacing with texttt{cellspace}}
{
    rowcolors{1}{gray!20}{white}
    begin{tabular}{QlQl}TopRule
        rowcolor{gray!40}
        My Head & makecell{My second\ Head} \ MidRule
        some content & $dfrac{1}{2}$ \
        foo & bar \ BottomRule
    end{tabular}
}\

tableComment{4: Lines with texttt{Xhline}, Head with texttt{thead}, spacing with texttt{makegapedcells}}
{
    rowcolors{1}{gray!20}{white}
    makegapedcells
    begin{tabular}{ll}TopRule
        rowcolor{gray!40}
        thead{My Head} & thead{My second\ Head} \ MidRule
        some content & $dfrac{1}{2}$ \
        foo & bar \ BottomRule
    end{tabular}
}
tableComment{5: Lines with texttt{booktabs}, Head with texttt{makecell}, spacing with texttt{booktabs}}
{
    rowcolors{1}{gray!20}{white}
    begin{tabular}{ll}toprule
        rowcolor{gray!40}
        My Head & makecell{My second\ Head} \ midrule
        some content & $dfrac{1}{2}$ \
        foo & bar \ bottomrule
    end{tabular}
}
tableComment{6: Lines with texttt{Xhline}, Head with texttt{thead}, spacing with texttt{cellspace}}
{
    rowcolors{1}{gray!20}{white}
    begin{tabular}{QlQl}TopRule
        rowcolor{gray!40}
        thead{My Head} & thead{My second\ Head} \ MidRule
        some content & $dfrac{1}{2}$ \
        foo & bar \ BottomRule
    end{tabular}
}

settowidthrotheadsize{theadfont second}
tableComment{7: Lines with texttt{Xhline}, Head with texttt{rothead}, spacing with texttt{callspace} }
{
    rowcolors{1}{gray!20}{white}
    begin{tabular}{QlQl}TopRule
        rowcolor{gray!40}
        rothead{My\Head} & rothead{My\second\ Head} \ MidRule
        some content & $dfrac{1}{2}$ \
        foo & bar \ BottomRule
    end{tabular}
}
tableComment{8: Lines with texttt{booktabs}, Head with texttt{rothead}, spacing with texttt{booktabs}}
{
    rowcolors{1}{gray!20}{white}
    begin{tabular}{ll}toprule
        rowcolor{gray!40}
        rothead{My\Head} & rothead{My\second\ Head} \midrule
        some content & $dfrac{1}{2}$ \
        foo & bar \ bottomrule
    end{tabular}
}
tableComment{9: Lines with texttt{Xhline}, Head with texttt{rothead}, spacing with texttt{makegapedcells}}
{
    rowcolors{1}{gray!20}{white}
    makegapedcells
    begin{tabular}{ll}TopRule
        rowcolor{gray!40}
        rothead{My\Head} & rothead{My\second\ Head} \ MidRule
        some content & $dfrac{1}{2}$ \
        foo & bar \ BottomRule
    end{tabular}
}
end{document}