Do the TW, THW and THHN or THWN wire insulation types matter in terms of powering RAMPS 1.4 or the MK2a Heat Bed?
THHN wire is thermoplastic high heat-resistant nylon coated wire.
THWN is thermoplastic heat- and moisture-resistant nylon coated wire.

"T" stands for thermoplastic insulation covering the wire itself.
"H" stands for a heat resistance of the insulation max 167°F.
"HH" stands for a heat resistance, but increased max 194°F.
"W" is for moisture resistant.
"N" is for a nylon coating make the insulation oil and gas resistant.

In my opinion the Wire Storehouse is good for simple stuff, but I would not use it for something I consider important.   It's quality is poor on the insulation rating and the number of strands is low.  Higher strands number allows for more flexible wire.

If you get high end audiophile type speaker wire it could be considered as you can find high strand number with good quality insulation properties in heavier gauges. 

Or your local auto parts store will carry 8 and 6 gauge wire with better insulation properties.