Try

(setq my-capture-template-next "* NEXT %?\n:PROPERTIES:\n:CREATED:
%U\n:END:\n\n")

(setq org-capture-templates
      `(  ; Note the backquote here!!
        ("1" "first version with string" entry (file+headline
"~/share/all/org-mode/misc.org" "shorts")
         "* NEXT %?\n:PROPERTIES:\n:CREATED: %U\n:END:\n\n" :empty-lines 1)
        ("2" "second version with variable" entry (file+headline
"~/share/all/org-mode/misc.org" "shorts")
         ,my-capture-template-next :empty-lines 1) ; and comma before
my-capture-..
        ))

My guess is you need to evaluate my-capture-template-next before putting it
in org-capture-templates and the backquote/comma above do that.
-- 

-- 
Kaushal Modi