I don't know where you are seeing that but the HD4870x2 should be the faster of those two setups. However the crossfired HD5750 will run much cooler and use much less power while giving you DX11 compatibility.
Personally I would sell both and just get an HD5850 and OC the hell out of it.
I'm really not interested in OC'ing my cards, as I would rather they last a significant amount of time.
If you could point me in the direction of the information about the 4870x2 that you are seeing. From where I sit, it looks like the 4870 is on the losing end of a 5750 Crossfire setup, not even counting the DX11 support and the Eyefinity.
OCing does not shorten the life span of anything unless you are pushing it to the limits into temperatures that are unwise.
As for the relative speeds it's the HD5770 that is on par with the HD4870(although the 128-bit bus does limit it slightly in crossfire) not the HD5750. The HD4870x2 is two HD4870s so it will be faster.
Basically it would go in this order in terms of average frame rates;
HD4870x2 > CF HD5770s > HD5870 > CF HD5750s > HD5850
It's hard to find reviews directly comparing those cards but most HD5870 reviews include the HD4870x2 and show it being slightly faster on average. Most reviews with crossfired HD5750s also include the HD5870 and show it being slightly faster.
I probably would. HD5750s should still perform very well and the power usage, lack of heat and DX11 are worth a lil less performance. You should be able to sell the HD4870x2 for a nice sum especially if the send you a new one.