I've seen this idea a few times, but I've never been too convinced by it. It's true that if you go up enough from the searing heat and crushing pressure of Venus' surface you'll find an atmospheric layer where temperature and pressure are quite similar to that of Earth's surface. Also a breathable oxygen/nitrogen mix would act as a lifting gas relative to Venus' atmospheric composition, so you could in principle float a big bubble full of air in it.
This does however gloss over a couple of issues. Firstly the amount of corrosive acid in the atmosphere is incredibly destructive to practically anything we can build. The record for keeping probes working on the surface of Venus still stands at a few hours. Moving to higher altitude means instead of sulphuric acid vapour, it's now just raining acid, which isn't much of an improvement. Secondly, wind speeds in this layer of the atmosphere can hit 500 mph, far above even the most intense hurricanes and tornados of Earth. I seriously doubt our ability to build something to withstand this longterm, and any kind of failure of this "bubble" means everyone inside is extremely dead.
There's also the issue that even if you could build such a floating city - what's the point? It's certainly going to be harder to maintain than a structure in plain old vacuum due to the extreme difficulty of accessing the outside of it. It's not like you can access much in the way of resources from Venus either. I suppose you could bring in some of the atmospheric gases, but it's not as if CO2 or sulphuric acid are particularly worth going there for. At least on Mars you could go outside your city with just a pressure suit.
As to the original question, sure I'd like to see a permanent human presence on Mars, but it's nowhere near as important as anything required for maintaining Earth. Realistically we might get a small, somewhat self sustaining little village there if we threw enough resources at it. The idea of Mars as an "escape route" or backup plan for catastrophe on Earth tends to fail to grasp that we'd be talking a few hundred survivors even if we're extremely optimistic. As to human expansion to other worlds, that's great - but if you're looking for return on investment, you'd better be looking over a timescale of millennia. It's not going to alter the situation on Earth in any of our lifetimes.