This is a quick and dirty script to compare two different s3-compatible buckets, just dorp in the bucket name and credentials, and optionally change the endpoint host

compare_buckets.py

#!/usr/bin/python

import boto, json

from boto.s3.connection import S3Connection
from boto.gs.connection import GSConnection

def compare_buckets(bucket_one_bucket_name, 
                    bucket_two_bucket_name, 
                    bucket_one_access_key_id, 
                    bucket_one_secret_access_key, 
                    bucket_two_access_key_id, 
                    bucket_two_secret_access_key, 
                    bucket_one_host="s3.amazonaws.com",
                    bucket_two_host="s3.amazonaws.com",
                    print_status=False, 
                    confirm_matches=False):
    '''
        This function compares the file contents of two S3-compatible
        buckets, skipping multi-part uploads, and returning files that
        are missing or that differ (via etag md5 hash).
        It may be helpful for migrating from S3 to GS.
    '''

    bucket_one_conn = S3Connection(aws_access_key_id=bucket_one_access_key_id, aws_secret_access_key=bucket_one_secret_access_key, host=bucket_one_host)
    bucket_two_conn = S3Connection(aws_access_key_id=bucket_two_access_key_id, aws_secret_access_key=bucket_two_secret_access_key, host=bucket_two_host)

    bucket_one_bucket=bucket_one_conn.get_bucket(bucket_one_bucket_name)
    bucket_two_bucket=bucket_two_conn.get_bucket(bucket_two_bucket_name)

    results = {}
    results["present"]              = []
    results["missing"]              = []
    results["different"]            = []
    results["skipped_bucket_one"]   = []
    results["skipped_bucket_two"]   = []

    bucket_one_keys = bucket_one_bucket.get_all_keys()
    numer_of_keys = len(bucket_one_keys)
    for i, bucket_one_key in enumerate(bucket_one_keys):
        if print_status:
            print( "Processing. {percent:.2%} complete.".format(percent=float(i+1)/float(numer_of_keys)) )

        if not bucket_one_key.name.endswith("/"):
            if "-" not in str(bucket_one_key.etag):
                bucket_two_key = bucket_two_bucket.get_key(bucket_one_key.name)
                if bucket_two_key:
                    if "-" not in str(bucket_two_key.etag):
                        if bucket_two_key.etag == bucket_one_key.etag:
                            if print_status and confirm_matches:
                                print("etag matches for:", bucket_one_key.name, bucket_one_key.etag, "==", bucket_two_key.etag)
                            results["present"].append(bucket_one_key.name)
                            pass
                        else:
                            if print_status:
                                print("etag differs for:", bucket_one_key.name, bucket_one_key.etag, "==", bucket_two_key.etag)
                            results["different"].append(bucket_one_key.name)
                    else:
                        if print_status:
                            print("multipart upload for file, skipping:", bucket_two_key.name)
                        results["skipped_bucket_two"].append(bucket_one_key.name)
                else:
                    if print_status:
                        print("object does not exist in GS:", bucket_one_key.name)
                    results["missing"].append(bucket_one_key.name)
            else:
                if print_status:
                    print("multipart upload for file, skipping:", bucket_one_key.name)
                results["skipped_bucket_one"].append(bucket_one_key.name)

    return results

if __name__ == "__main__":
    bucket_name = "bucket-name"
    results = compare_buckets(
                    bucket_one_bucket_name        = bucket_name, 
                    bucket_two_bucket_name        = bucket_name, 
                    bucket_one_access_key_id      = "", 
                    bucket_one_secret_access_key  = "",
                    bucket_two_access_key_id      = "",
                    bucket_two_secret_access_key  = "", 
                    bucket_two_host               = "storage.googleapis.com",
                    print_status                  = False, 
                    confirm_matches               = False)

    print(json.dumps(results, sort_keys=True, indent=4, separators=(',', ': ')))