I have looked at the Sparc but yet to order one. I have bought 3 of the Strikefire II's and I love them. I picked them up for 175 each. I also bought the Lucid HD7 and it is also a nice unit. It is around the same price of the Vortex.
http://mylucidgear.com/reddotsights.php
http://mylucidgear.com/reddotsights.php
http://mylucidgear.com/reddotsights.php