We have this one, which is what I'd consider the gold standard. It's ISOFIX, and only rearward-facing since kids under 4 should not be facing front, and it's won basically every test there is:
http://www.besafe.com/product-int/izi-kid-x2-i-size/
It weighs 40.6 lbs. Quite a bit of structural steel in there, including a full integral roll bar and a brace against being rear-ended, both of which your linked Britax seat lacks.
Also bloody expensive, but if there's one thing I won't skimp on, it's the safety of my children.