The H=28 medium rails (20mm wide) are more than strong enough, even if you machine steel regularly. Their dynamic load rating is 7.2 kN, or 1600 lb. "Dynamic load rating" refers to how much load the blocks can safely carry while moving. (Technically, it's the load that 90% of the blocks can carry without failure for some long distance, typically 50 km for profile rail.)
For a lower Y rail, assuming the cutter tip is 180mm (7") below that rail, the Y rails are spaced 120mm (4.7") apart, and there are 2 bearing blocks per rail, it would take a longitudinal cutter force of 1300 lb to put 1600 lb on each lower bearing block. (See the xls calculator here, using AB=4.7 and BC=7:
http://www.cnczone.com/forums/diy_cn...ml#post1408004).
If you only had 1 bearing block per rail, the high torques on each block might eventually wear them out. But 2 blocks per rail is important for stiffness, so there will be no issue for load capacity.
The smaller H=24 (15mm wide) rails would be fine, too, if the screw sizes were acceptable (usually they're quite small for 15s).