But it doesn't compare to an SSD. You say speed, meaning performance, but you compare a MomentusXT to say my Crucial SSD, and it'll leave the MomentusXT for dust.
The only performance increase I can see is the difference between a traditional fully mechanical 7200rpm hard drive.
For now a hybrid might might sense, but given a few years you'll be seeing 1TB SSD capacities offered at moderate prices, alongside multiple terrabyte (like 10+) hard drives.
A 256GB SSD is plenty big enough for most users, even when considering games, game saves, the OS, software and personal files and media.