Of the three places you mention, I've stayed only at Luba Hati (a few weeks ago) though I've visited Inn at Roberts Grove a number of times and Mata Chica twice, and I've eaten at Robert's Grove and Mata Chica.
Of course the basic decision is whether you want to go to Placencia or Ambergris Caye. This issue is discussed frequently on another Belize board, so I won't go into the pros and cons of each area.
All three places you mention are excellent. In our Belize Book of Lists 2000 (available from Amazon.com and elsewhere) we rate Inn at Robert's Grove the # 1 mainland seaside resort and Mata Chica one of the top two or three resorts on Ambergris Caye.
Another place to consider on (or rather near) Ambergris, which is really romantic is Cayo Espanto. It's four villas on a private island on the back side of Ambergris. You get your own "butler," delicious meals delivered to your villa, and the villas themselves are beautiful designed with all the modcons including stereos and cable TV, Egyptian cotton sheets, small private pools (they call them "splash pools.")
The only downside is that it's very expensive -- like US$600-$1000 or more a day, but that includes meals, and they usually have some off-season deals.
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Belize First Magazine http://www.turq.com/belizefirst/
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