Not for lack of trying, I haven't managed to find any sleazy nightlife. Perhaps you would be good enough to share your haunts with me?

Ha! We are 28 and 31 and a couple so not really going for sleezy nightlife, but some nightlife would be nice...!
Mookie knows all the haunts, joints and hang outs of the Hanoi area, I got the grand tour and had a hangover for a week. Bring Tylenol!
There isn't really sleazy nightlife in either city. I'd say by your ages you are more likely to prefer HCMC but there are different things in each city. The Hanoi/HCMC question appears a lot so I'm gonna sort it out once and for all:
In Hanoi they have Ho Chi Minh's corpse on display so if cadavers are your thing then this is the obvious choice. If you want to go to Halong bay then Hanoi is the natural choice too. Also, you are unlikely to find me in Hanoi, so as mrmookie will attest - this makes it generally more enjoyable for everyone else.
HCMC, the weather is warmer, some say the people are warmer. It's a bigger city but deinitely has a more provincial feel, though the traffic can be stifling. The fruit here is extemely plentiful and fresh due to the city's proximity to the delta, but dog meat is harder to find than up north.
That about sums both cities up. I doubt there's anything anyone could add to that.
Its more likely the women in Sai Gon dont wear underwear than the women in Ha Noi.....hows that for sleezy factor?