UPDATE: I'm an idiot. I was referring to the Quantum LM the whole time. The AS is a totally different drive!!!! If you're getting Quantum, the LM is the one to consider.
I have the 30GB version of both drives right now. They currently both hold the IDE speed crown. There is little noticeable performance difference, but I'd say Quantum wins seektime-dependent applications (program loading times, databases, etc) and IBM wins transfer rate-dependent ones. Check Storagereview.com's database.
Apart from speed, the IBM is definitely quieter; the Quantum produces a whine. With both disks inside a SilentDrive enclosure, only the 75GXP's spinning noise is virtually eliminated. Seeks are noticeable on both (but Quantum [edit:]LM's are the best of any IDE disk), however IBM's acoustic software allows silencing seeks at the expense of seek times. Heat isn't a problem for either disk, since both work fine inside a SilentDrive.
If you want SCSI-like performance and don't worry about noise, get the Quantum LM 30GB. It's unreasonably cheap and has ~7.2ms measured seek times, and the best IDE workstation/server scores on Storagereview. If you rather prefer quiet operation and fast sequential transfer rates, the 75GXP is the winner. I'm confident in the quality of both.