Otdfbt

Consider this code in the TikZ - PGF manual, which uses a macro

% Main code from
% The TikZ - PGF manual
% Author: Till Tantau et al
% Version 3.1.3, released May 9, 2019
% Page 40
documentclass[tikz]standalone
begindocument
begintikzpicture
 defrectanglepath-- ++(1cm,0cm) -- ++(0cm,1cm) -- ++(-1cm,0cm) -- cycle
 draw (0,0) rectanglepath;
 draw (1.5,0) rectanglepath;
endtikzpicture
enddocument

and this one written by me, which uses pic

documentclass[tikz]standalone
begindocument
begintikzpicture
[rectanglepath/.pic=draw (0,0)--++(1cm,0cm)--++(0cm,1cm)--++(-1cm,0cm)--cycle;]
 pic at (0,0) rectanglepath;
 pic at (1.5,0) rectanglepath;
endtikzpicture
enddocument

Both give us the same output

Observation

Both give the same output. I prefer the latter one, as it is more "TikZ-ish". However, we can always change a code from pic to macro and vice versa, as above (am I wrong – is there a case in which we can't convert?), or draw the same figure using two different codes (this and this for example).

Also, I can't see any aspects in which macros are better than pic. Even arguments: we can also have pic with up to nine arguments with any pattern using /.style args.

In the PGF manual, sometimes I see pic is used, but sometimes a macro is used (it is even used in the title page!). From those examples, I can't figure out in what case I should use a macro and in what case I should use a pic.

Question

It may be quite clear by now.

If I have to draw the same "sub-"picture many times in a TikZ picture, should I use a pic or a macro? And why? Is there any cases in which I must use this one and not the other?

Thanks in advance.

Edit

As for @marmot's nice codes, I have written myself some code (against it) which uses macros

documentclassarticle
usepackagetikz
begindocument
subsection*Macros can have more complicated constructions

begintikzpicture%[rectanglepath/.pic=draw (-0.5,-0.5) rectangle ++(1,1);
%draw[red] (-0.5,0.5) -- (0.5,-0.5);]
newcommandrectanglepath[2][]scope[shift=(#2),#1]draw (-0.5,-0.5) rectangle ++(1,1);
 draw[red] (-0.5,0.5) -- (0.5,-0.5);endscope
%path (0,0) picrectanglepath (2,0) pic[dashed]rectanglepath;
rectanglepath0,0
rectanglepath[dashed]2,0
endtikzpicture

subsection*Macros can have names

begintikzpicture%[rectanglepath/.pic=draw (-0.5,-0.5) rectangle ++(1,1);
%draw[red] (-0.5,0.5) coordinate(-tl) -- (0.5,-0.5) coordinate(-br) ;]
newcommandrectanglepath[3][]scope[shift=(#2),#1]draw (-0.5,-0.5) rectangle ++(1,1);
 draw[red] (-0.5,0.5) coordinate (#3-tl) 
 -- (0.5,-0.5) coordinate (#3-br);endscope
%path (0,0) pic (A) rectanglepath (2,0) pic[dashed] (B) rectanglepath;
rectanglepath0,0A
rectanglepath[dashed]2,0B
draw (A-tl) to[out=30,in=150] (B-tl) (A-br) to[out=-30,in=-150] (B-br);
endtikzpicture

subsection*Transformations are much more intuitive on macros

begintikzpicture[rectanglepath/.pic=draw (-0.5,-0.5) rectangle ++(1,1);]
newcommandrectanglepath[2][]
 scope[shift=(#2),#1]draw (-0.5,-0.5) rectangle ++(1,1);endscope
%path (0,0) pic[rotate=45]rectanglepath (2,0) pic[dashed,rotate=-30]rectanglepath
%(4,0) pic[thick,xscale=0.5]rectanglepath;
rectanglepath[rotate=45]0,0
rectanglepath[rotate=-30,dashed]2,0
rectanglepath[thick,xscale=0.5]4,0
endtikzpicture
enddocument

Here are threefour examples of things that are much harder to achieve in the macro approach. The first three examples have been translated to macros in the revised question.

documentclassarticle
usepackagetikz
begindocument
subsection*Pics can have more complicated constructions
tikzsetrectanglepath/.pic=draw (-0.5,-0.5) rectangle ++(1,1);
draw[red] (-0.5,0.5) coordinate(-tl) -- (0.5,-0.5) coordinate(-br) ;

begintikzpicture
path (0,0) picrectanglepath (2,0) pic[dashed]rectanglepath;
endtikzpicture

subsection*Pics can have names

begintikzpicture
path (0,0) pic (A) rectanglepath (2,0) pic[dashed] (B) rectanglepath;
draw (A-tl) to[out=30,in=150] (B-tl) (A-br) to[out=-30,in=-150] (B-br);
endtikzpicture

subsection*Transformations are much more intuitive on pics

begintikzpicture
path (0,0) pic[rotate=45]rectanglepath (2,0) pic[dashed,rotate=-30]rectanglepath
(4,0) pic[thick,xscale=0.5]rectanglepath;
endtikzpicture

subsection*You can insert pics in paths

begintikzpicture[xboard/.style=insert path=
(0,0) grid (#1,#1)
foreach X in 1,...,#1
(X-0.5,X-0.5) picrectanglepath (X-0.5,#1-X+0.5) pic[rotate=90]rectanglepath]
draw[xboard=1,xshift=2cm,xboard=3,xshift=4cm,xboard=5];
endtikzpicture

enddocument

My personal take: pics win, at least in the above-described scenarios. IMHO the translated macros in the revised question substantiate this.

Notice that "behind the scenes" pics are macros at some level. So you will always be able to reproduce the outcome of a pic with a macro. However, the effort will be more substantial. And yes, if you are in the mood to do some unnecessary extra work, you can use macros instead of pics. So, yes, the choice can depend on the mood of the day.

Let me also stress that there are scenarios in which pics perform worse. An example of this type may be a life wheel.

BTW, there is also the insert path option that is sometimes preferable over pics IMO.