The Ground Control HD2 frame walls are thicker and stiffer than the originals, increasing both overall strength AND energy transfer. Countersunk axle holes help to protect bolts from wear.
Comes with both plastic and aluminum h-blocks so that you have options to grind on all kinds of obstacles. The same size h-blocks are used on the GC 60mm HD2 frame as the 72mm HD2.
Sizing is by personal preference. A longer wheel base is more stable and shorter is more agile but, as a guideline, sizes 9.0+ often choose large or extra large.