Well, from what i have been informed, the GT40 is a good towing/daily driver turbo. Yet it is still capable of making boost as high 45 psi. The GT42 is much bigger, and harder to spool up, so it is more suited to racing, and maybe some street driving.
Well, I have the GT40, peak boost: 46, egt on 1/4: 1,500*, lag: almost nonexistant, failures: none. its a great turbo, and hopefully someone can tell you about the GT42