with radiators (heatercore style ie. multiple parallel channel 1 or 2 pass radiators such as a black ice) simply bigger is better, the larger the surface area the better the cooling, and hopefully given the parallel channels most heatercores will have simalar (very low) flow restriction. to get bigger surface area you either get a physically bigger heatercore, or you get a heatercore with more folds per inch (fpi).
the black ice extreme has 16fpi, which is abotu average, the 6x6x2 popular heatercore has 12fpi, heatercores are known to go up to 24fpi as far as im aware (the toyota camry heatercore is 24fpi). also the black ice extreme is physically small, and although it is a heatercore style radiator, so it is good for its size, a much larger surface area radiator can be up to 2 degrees better.
the waterblock you use will depend on the pump (and pipe?) size you use. a waterblock with a large water channel area will work better with a high powered pump than a waterblock with a narrower channel, and alternatively with smaller pump, a block with a narrower water channel will work better than a block with a wide water channel.
for example a spir@l which has a pretty large water channel will need a big pump for optimum results. i will not speculate on big a pump this needs to be, this could not be determined without tests, although perhaps a little giant pump of some kind would be perfect?
a tc-4 i think (correct me if im wrong) has a slightly narrower water channel than a spir@l, so it should be more suited to a smaller pump, perhaps an eheim 1250 or equivalent. it still may perform slightly better with a bigger pump, however this difference will not be as great compared to the difference noticed when using a bigger pump on a larger channeled block.
of course, this is assuming the blocks have the same design, which they clearly do not, so there will be many other factors effecting the differences between two temperatures, so in fact one or the other could perform better for all pump sizes.
still, channel width is one very significant factor when considering a waterblock, if you want to get that pump that is normally found in a high pressure cleaner
then consider a large channeled block such as a spir@l, if you would prefer a smaller eheim 1048, then a small channeled block,such as a gemini block would be better suited.
basically just dont get a huge pump for a gemini, or a piddly pump for a spir@l, get the pump to match your block, or the block to match your pump