I enjoy my Master Replicas Vader helmet, as well as my LE Vader sabers. They are some of the highlights of my collection. However, I also really enjoy the props I've obtained from prop makers off of the RPF and Rebelscum, sometimes for MUCH less than the high markup of companies like Master Replicas. So I can see why you would choose EITHER route.
One reason why I changed my mind about the eFx McQuarrie Vader they are planning to release. I originally was excited about this helmet and couldn't wait for its release. I was in constant contact bugging Amy on updates to its release, before she quit eFx. It irritated me to no end that the piece was slated to be sold in February 2009 and we still yet to have heard word about it. Then a couple of months ago, I can upon Stone's Concept Vader that I linked which is MUCH MORE accurate to the original Vader designs. (Which is quite odd, because McQuarrie himself had the opportunity to look over the first molds of the eFx Vader and gave his blessing to my understanding. Vader's original designs had a feel of a cross between cybernetic and insect when looking at him, mixed in with Samurai. Of course, due to Lucas influence from the Japanese genre which would be later utilized in filming) eFx's helmet is expected to be at around $1,000+. Stone's, for $550 plus shipping, I would be getting an awesome cast from a excellent prop maker on the RPF, which to many peoples' eyes is MORE accurate and better constructed than the eFx one is expected to be.
That deal that Wedge found you for the Boba SE is AMAZING and if I didn't have so much money already directed towards other endeavors (2 other Vader Helmets to add to my collection, the life-size Vader replica I"m working on, and my Han In Carbonite desk) I would definitely jump at it myself.
I think she would kill me if I also went for the Boba Fett. She would be like "But your not even a die-hard Fett fan?!?" My powers of the Dark Side are strong....yet even I answer to my Emperor. LOL...
In the end, make sure you are getting pieces that you will enjoy. If you like the Fett SE, then go for it. If you would like to get a Mando Helmet that you can customize, PLUS afford to get another piece, then go for it. In the end, if you choose from purely an enjoyment perspective, you'll never go wrong.
If you can make one-heck-of-a-deal WHILE purchasing that piece you'll love like I did on my MR Vader lid, then even better. You get to sit back and bask in the beauty of the piece, and also feel great about the hundreds you've saved at the same time. ;D