Whats the difference between a Serial ATA and a Ultra ATA ide HD's? Which is better?

The difference is a leap in technology.

Ultra ATA is the final standard of Parallel ATA (PATA), capable of running in 66 MB/s, 100 MB/s or 133 MB/s. SATA is the new standard that runs at 150 MB/s. SATA II is the natural evolution, running at 300 MB/s. SATA also has a number of features such as NCQ (Native Command Queuing) which reduces the amount of work a Hard Drive has to do, Hot plugging (controller support dependant) allowing Hard Drives to be added and removed without rebooting, and more.

PATA is as the name suggests, a parallel interface, meaning it is composed of a cluster of wires that carry synchronized data in parallel. By raising the speed of the wires, the bandwidth is raise, but that presents the issue of electromagnetic cross-talk; where the magnetic field produced by an electron running down a wire influences the next wire. Any interference on any of the wires effective corrupts the entire transfer, and it has to be done again (this occurs invisibly; the system doesn’t crash or give an error, but it slows things down). With the ATA 133 standard, additional grounding wires were added to reduce this effect. You may have heard of 40 and 80-conductor cables (sometimes incorrectly referred to as 40 and 80-pin; if you check an 80-pin cable it only has 40-pins); the extra 40 wires, or conductors, are for the grounding.

SATA (Serial ATA) is a serial interface. This means it can run many times faster without causing any self-inflicted interference, which was not previously possible due to technological constraints. SATA is superior in every respect; both bandwidth and features. Furthermore it uses a low profile cable that is easier to route inside the case. The huge bandwidth increase doesn't translate directly into Hard Disk performance however, since the actual data transfer rate is limited by the HDD technology. In many cases, the actual SATA HDD assemblies are exactly the same as their PATA counter parts, only the controlling logic is different. In these examples, the maximum data speed might only be 60-70 MB/s off the platters; that extra 166 MB/s of peak bandwidth that SATA II offers over ATA 133 is only achievable in cached transfers.