I havent heard many problems with either drive besides a few of the Barracuda IV's having a high pitched whining noise, but those have mostly been fixed via RMA and all of the new one's are fine.
Both manufacturers are generally reliable, though personally I'd give the edge to Maxtor.
The Barracuda IV is somewhat quieter, while the Maxtor D740X is slightly faster... and the D740X supports ATA133 if you care.
The Barracuda IV tends to have an abnormally large performance drop when running in ATA33, so if your going to be utilizing the drive in an old ATA33 mobo it will be a decent amount slower then the D740X in ATA33.... but that's a very minor issue since any semi-recent board has at least ATA66.
The Barracuda IV also doesnt tend to take to well to RAID setups, but again that's a relatively minor issue.
Both drives seem reasonably capable of taking an out of spec PCI bus, up to 37.5MHz or so.
I've got the Maxtor D740X (Fluid Dynamic Bearings model), and have been very happy with it. Even with FDB it's still slightly louder then the Seagate Barracuda IV though.
IMHO it basically comes down to this:
If you want performance: Maxtor
If you want low noise: Seagate
If cost is an issue: Seagate, as it's slightly cheaper and pretty close in terms of performance.