I own the 5970 and I've never had problems with it. It's a good card and plays even the most modern games on high settings; so I really have no reason to upgrade.. but then again even if I did, investing into old technology is usually not the right way to go. I like the 7970 for what it is, and what it is... is a very epic single-gpu beast. I like that single-gpudness of it because with dual gpu cards there are always drive related issues and things ain't scaling well etc.
And as for your statement that the 7970 is slower, give atom some time so that he gets the kernels coded for the HD7xxx series cards! I bet they'll be at 5970's level of greatness.
And just wait for the dual-gpu card that's sure to come soon!
TL;DR if your only goal is cracking open those hashes and you have access to cheap 5970s, get them. Otherwise, don't really invest in old tech and wait for the dual-gpu 7970 and get that instead!
How long do you think till AMD releases a linux driver?
EDIT: Oh I forgot to add... my 5970 clocks like mad! It comes with some boring clocks like... 725MHZ on the cores or something. Well, if you get yourself MSI afterburner tool and raise the voltage of the core to 1.162v, you can make the card do scary things. Mine's running @ 900Mhz at the moment. 900! The performance boost in oclhashcat-lite is very noticable. I can't remember it exactly but it was about 15 million more variations per second, where my previous number was hovering around 68M (if you think these numbers are low, it's because I am running brute-force on a DES cipher, not md5.). I read stuff about the 7970 and that it can be clocked nicely too but just saying you'll definitely get your money's worth if you get yourself that 5970. I've been abusing mine for almost 2 years on the stock cooler and it never even went over 88C. Yeah the fan gets noisy but who cares