having trouble deciding, any help with britax convertibles?

i get so confused with the diplomat, marathon, roundabout.......which is the safest convertible carseat from britax? can someone give me a link on the safety study that states why one is safer than another?thanks in advance...

According to Consumer Reports [which my husband subscribes to] it showed the Roundabout as being the safest! SO, we bought the Roundabout.

After buying the Roundabout I realized that the Marathon goes up about 25 lbs. more than the Roundabout, so it's a better investment financially. But as far as 'safety' goes, the Roundabout got the highest ratings.