Other than the difference between the form factors, it seems to me that the concepts are similar, and so I don't really know why OCZ decided to develop this product, let alone add a premium price over the existing Seagate hybrids.
The difference is OCZ is taking their Sandforce 2200 series Revo super drive and adding an hdd for 'storage space'.
If you are in the market for pci-e based ssds you are not likely to drag it down with a spinner. imho.
The momentus hybrid is just a spinner with a 4gb nand cache. Any of your most read documents will cache in the nand and come up 'ssd like' when you start them.
The difference is day and night. The momentus is a nice little bridge between hdd and ssd storage. This OCZ thing is trying to take a super fast ssd and sell it as a 1tb superfast ssd (oh and that 1tb is spinning btw).