If Middlebrooks was assured of everyday at-bats, I'd like him over Moose, mostly because I think he'll have more run-producing opportunities in the BoSox lineup. Unlike the previous poster, I see their upsides as very similar. But WM is not assured everyday at-bats -- until Youkilis gets traded (and he will be).
So, for now, I'd probably drop Middlebrooks for Moustakas. Hold Freese.
I would not hang onto any player waiting for a trade. It is a matter of time before Hosmer starts hitting and that will help Mous. Mous has always been a better prospect than Middlebrooks, but I agree it is much closer if they both played everyday (based on the lineup and home park). All that said go with Mous.
I live in Boston and picked up Middlebrooks about two weeks before he got the call-up. He was killing it down in Pawtucket, so a promotion seemed likely, and I wasn't going to miss out as I had been looking for a decent 3B replacement while Longoria is out. As soon as Youkilis hit the DL he got his shot. Yes, Moustakas is the more highly touted prospect. But who cares? Ultimately that means nothing. Zero, zilch, nada. It's all about production. Yes, Middlebrooks has a big swing and will strikeout a bit, but he can also flat out rake. Word is that a deal was going to happen that sent Youk to San Francisco, but I guess the Sox wanted too much in return. Oakland is always a possibility, too. Beane and his OBP cohorts love "The Greek God of Walks" as much as anyone. I'm in a 12 team H2H Keeper League and I personally would rather have Middlebrooks. Just from watching him briefly thus far I think he's got it. But then again, Moustakas probably does also. And the fact that Will may be sitting more than not from here on out is reason for concern. At least in the short-term. He can't produce while riding the pine. But I think, as others above me have already pointed out, that Youk will be dealt and the everyday 3B job will be handed over to Middlebrooks. The future is now. Worst case scenario Middlebrooks produces here and there sitting on my bench and then gets the gig next season when the Sox neglect to pick up Youkilis' extension. Be patient if you can. If not, and you absolutely need the help right now, go with Moustakas. But I have a feeling come August you'll wish you had held on to Will.
Go with Moustakas for now. Don't count on Youk getting traded. If he does, it'll be when the BoSox fall out of the race, and will not rush back Ellsbury and Crwford, and Ryan Sweeney is not a top OF providing RBI opportunities.
I also agree that you should grab Moustakas. Mean power bat. 36 homers in one minor league season two years ago, which is ridiculous given that minor league stats are usually lower than major league stats. That and he's more highly developed and guaranteed a roster spot.