What are some benchmarks with Mysql vs MariaDB while on Drupal 7?
When you have applied full due diligence, the difference between MariaDB and MySQL should be virtually negligible. Why would I say this?

MariaDB has done some benchmarking with some settings that give all MySQL processes the same level playing field : See Sysbench OLTP: MySQL-5.6 vs. MariaDB-10.0.  In fact, here were the setting used in the benckmark.

[mysqld]
sql-mode="NO_ENGINE_SUBSTITUTION"
back_log=500
user=root
port=3306
max_connections=4096
max_connect_errors=5000
max_prepared_stmt_count=50000
table_cache=2048
transaction_isolation=REPEATABLE-READ
loose-skip-external-locking
innodb_status_file=0
innodb_data_file_path=ibdata1:200M:autoextend
innodb_buffer_pool_size=1G
innodb_additional_mem_pool_size=20M
innodb_log_file_size=650M
innodb_log_buffer_size=100M
innodb_support_xa=0
innodb_doublewrite=0
innodb_flush_log_at_trx_commit=0
query-cache-size=0
query-cache-type=0
symbolic-links=0
skip-grant-tables


Please note


the settings do not accommodate use of more read and write I/O threads.
double write buffer disabled, good for speed, bad for data consistency in case of a crash




The key to really deciding which version of MySQL to choose is to effectively tune all the versions  under test and then benchmarking with actual Drupal data. I would look into getting all MyISAM converted to InnoDB as much as possible (If you are using FULLTEXT indexes, you must go with MySQL 5.6).

Here are some of my past posts from the DBA StackExchange on what settings to look for when tuning InnoDB and why:


Jun 19, 2011 : How do I properly perform a MySQL bake-off?
Nov 24, 2011 : Why mysql 5.5 slower than 5.1 (linux,using mysqlslap)
Oct 05, 2011 : Query runs a long time in some newer MySQL versions