Comparing Closure Compiler and UglifyJS2 http://www.peterbe.com/plog/advanced-closure-compiler-vs-uglifyjs2

compare.py

import StringIO
import gzip
import os
import subprocess
import time


def compiler(path):
    cmd = 'java -jar compiler-latest/compiler.jar -O advanced'.split()
    cmd.append(path)
    t0 = time.time()
    output, error = call(cmd)
    if not output.strip():
        # When you use `-O advanced`
        cmd = 'java -jar compiler-latest/compiler.jar -O simple'.split()
        cmd.append(path)
        output, error = call(cmd)
        error = 1
    t1 = time.time()
    return output.strip(), t1-t0, error


def uglify(path):
    cmd = ['./node_modules/.bin/uglifyjs']
    cmd.append(path)
    cmd.append('--mangle')
    cmd.append('--screw-ie8')

    t0 = time.time()
    output, error = call(cmd)
    t1 = time.time()
    return output.strip(), t1-t0, error


def gzip_string(s):
    out = StringIO.StringIO()
    with gzip.GzipFile(fileobj=out, mode="w") as f:
        f.write(s)
    return out.getvalue()


def call(seq):
    if isinstance(seq, basestring):
        seq = seq.split()
    return subprocess.Popen(
        seq,
        stdout=subprocess.PIPE,
        stderr=subprocess.STDOUT
    ).communicate()


def run(*args):
    paths = args[0].splitlines()
    import re
    paths = [x for x in paths if not re.findall('\.[a-f0-9]{12}\.', x)]
    # import random
    # random.shuffle(paths)
    # paths=paths[:3]
    print len(paths), "FILES"
    print "PATH".ljust(30),
    print "SIZE".rjust(10),
    print "CLOSURE".rjust(10),
    print "(GZIP)".rjust(10),
    print "(TIME)".rjust(10),
    print "UGLIFY".rjust(10),
    print "(GZIP)".rjust(10),
    print "(TIME)".rjust(10),
    print
    before_total = total = utotal = gtotal = ugtotal = 0
    ttotal = uttotal = 0.0
    for path in paths:
        result, t, errors = compiler(path)
        assert result, path
        uresult, ut, uerrors = uglify(path)
        assert result, path
        p = os.path.basename(path)
        if errors:
            p += ' *'
        if uerrors:
            p += ' ^'
        print p.ljust(30),
        before = os.stat(path).st_size
        print str(before).rjust(10),
        print str(len(result)).rjust(10),
        print str(len(gzip_string(result))).rjust(10),
        print ('%.2fs' % t).rjust(10),
        print str(len(uresult)).rjust(10),
        print str(len(gzip_string(uresult))).rjust(10),
        print ('%.2fs' % ut).rjust(10),
        print
        before_total += before
        total += len(result)
        utotal += len(uresult)
        gtotal += len(gzip_string(result))
        ugtotal += len(gzip_string(uresult))
        ttotal += t
        uttotal += ut

    print "TOTAL".ljust(30),
    print str(before_total).rjust(10),
    print str(total).rjust(10),
    print str(gtotal).rjust(10),
    print ('%.2fs' % ttotal).rjust(10),
    print str(utotal).rjust(10),
    print str(ugtotal).rjust(10),
    print ('%.2fs' % uttotal).rjust(10),
    print

if __name__ == '__main__':
    import sys
    sys.exit(run(sys.stdin.read()))

1. Download the compiler from: https://dl.google.com/closure-compiler/compiler-latest.zip (link from https://developers.google.com/closure/compiler/)
2. run npm init and npm install --save uglify-js (or just npm install -g uglify-js if you want to be global)
3. Pick some directory that has (recursively) lost of your .js files and run it something like this:

find /path/to/my/project/ | grep '\.js$' | grep -v '\.min\.'  | python compare.py

If you can, try to avoid directories such as node_modules or bower.