I have a St. Croix M casting rod and a G Loomis MH Casting rod.  While the G Loomis is a MH it is rated for the exact same lure weight and line size as the St. Croix M.  Is this true through out both brand of rods?  

I have a St. Croix ML rod and have read that St. Croix's are typically underrated. So, my rod would be considered by other manufacturers as a M rod. So your Croix M is more like other's MH.

I've found it to be true in both spinning and casting bass rods. St. Croix has Medium-Light powers in some rods, and these are very close to Loomis mediums as far as recommended line and lure weights.

Power ratings vary among the various series within the same brand. I fish both St. Croix and G.Loomis.

As a "general" overview, I think St. Croix Medium Power ranks very close to "most" G.Loomis Medium Heavy. However my Medium Light St. Croix (Avid AS66MF) is almost identicle to a G.Loomis "2" rated blank in the Bronzeback series. The ML seem comparable to me.