When I last looked, the two icons were identical. Curiously, if you make an alias using the code for making bookmarks, they contain the icon only once, so they are much smaller -- yet as far as I can see the files behave identically. But one icon might be used for high-res displays now -- I haven't checked.
I actually use that in ASObjC Runner's "manage file ... with creating alias" command, so you can play with it yourself. Or you can use the longer version to see what's happening:
script makeAlias
set thePath to current application's NSApp's passedValue() as text -- get passed path
set originalURL to current application's NSURL's fileURLWithPath_(thePath) -- make NSURL of it
set aliasURL to current application's NSURL's fileURLWithPath_(thePath & " alias") -- make NSURL for alias
-- build bookmarkData
set bookmarkData to originalURL's bookmarkDataWithOptions_includingResourceValuesForKeys_relativeToURL_error_(current application's NSURLBookmarkCreationSuitableForBookmarkFile, missing value, missing value, missing value)
-- write it to file
set theResult to current application's NSURL's writeBookmarkData_toURL_options_error_(bookmarkData, aliasURL, 0, missing value)
end script
set thePosixPath to POSIX path of (choose file)
tell application id "au.com.myriad-com.ASObjC-Runner" -- ASObjC Runner.app
run the script {makeAlias} passing thePosixPath
end tell