I think you will like either one, but the question is of what you want to do with them.  For higher precision work and no hand switching CQB stuff the Sierra Precision is awesome, very comfortable, fills the hand nicely and locates the trigger finger very well.  But of course with that big shelf on the left side if you ever needed to use your left hand on the PG it might make it a wee difficult.  
The G27 fits my hand great and the textured finger grooves make a very easy to control package, for me.  The thing I didn't like about my Ergo grip was the thickness, or lack thereof and the G27 is about a 1/4" wider all the way down and doesn't taper like the ergo.  I have a large hand and the G27 fits me better than any other grip I've used so far other than the Sierra Precision, but still is ambi and I think a little more useful than the SPR for general rifle use.  Of course, Wes at MSTN is putting the SPR on all their guns and they are going over great, so it's all in the eyes of the user.  Good luck.

Jason