they appear to be the same system except the big water 745 has a much larger cooling capacity than the 735, i would say more than double going by the physical dimensions. I like the 745 better because of the additional external heat exchanger (HX), which is more than likely plug and playable into the 735 system (they use the same pump). I would expect a lower rate of flow through the larger system (though it says they are capable of pumping the same amount) due to the additional pressure drop in the larger HX. They still used forced convection which leaves you with a smaller, but noisier system compared to the symphony, but the 745 will probably keep u REALLY cool. i would spring for the 745 if i was going that route.
NOTE: You still have the problem of a single pump. if you'r using the 745, you'r prob going to be OC'ing like crazy. if your pump goes out, you could fry your CPU in < 1minute. thats why i am a major supporter of a parralel pump system.