Laser issues were fairly common among the early 60G and 40G PS3s. In some cases, the BD drive failed completley, I have a spare 60G unit like that right now. Does the 40G have the latest firmware? If it is a decoding issue with certain discs, that is usually fixed via firmware.
Do not use disc cleaners. Blu-rays have a scratch resistant coating and unless you toss your discs around without a case, scratching isn't as likely of an issue. I own almost 300 BDs and have never had a single one have an issue with them being scratched, though I have had one or two discs that were defective.