The Barracuda Green 2TB is an attempt by Seagate to crack the low power consumption and excellent acoustics of low power desktop drives from WD and Samsung. The Momentus 750GB on the other hand doesn't have much to prove, coming from the same stock as the surprisingly quiet Momentus 7200.4 and XT.
Slow computer got you down? Maybe a hardware upgrade should be in your cards, if so this guide to the Fundamentals of Upgrading a PC is an invaluable read. PCSTATS Tips