For F2P players there is not much choice between different amulets. There are however two great ammys. Amulet of Power or Amulet of Strength? Amulet of Strength is the cheaper one, and a lot more popular around lower levels, but is it better?
Above is a stat-comparison – thanks to runescape.wikia.com – (click to zoom) which clearly shows that the amulet of power is better except for price, and strength. As you know, for a warrior strength is very important and 4 points less can be a lot for your maximum hit. The problem is, the power amulet also gives attack bonus! So yes, your max hit will be reduced but thanks to +6 in attack you will hit more often!
Personally, I would go for the Amulet of Power. This includes strength pures. It cost a little more, but gives defence bonus (very useful for pures) against ANY kind of opponent AND you get an additional attack bonus. So run to the Grand Exchange and forget about the two extra K!