Is an application like armory or bitcoin-qt more secure/ trustable in generating the public and private keys than a locally installed version of bitaddress.org ?
There are different issues with generating cryptographic keys in general (including private keys for bitcoin) on a downloaded client versus a client served online, as bitaddress.org does. However, I think you yourself should be the judge of what is "more trustable." Here are the issues:
Quality of random number generator
Opportunity and effort vs. reward trade-off for an adversary
Sometimes it is more dangerous to go the same route as everybody else by presenting, collectively, a larger incentive to exploit this route. Sometimes it could be more dangerous to do the opposite and possibly ending up using a solution that has received less public scrutiny and might, without one's knowledge, contain critical vulnerabilities.
My first instinct is to answer "No". As long as an application is installed locally and cannot send information over the internet to a third party, it's pretty much as secure as can be. There's no good reason to think that either would be more secure.
But there is a slightly bad one... (maybe not bad, but possibly a bit paranoid)...
Consider this: even if a program is open source, it's compiled, which means it not feasible to see the actual code and what it's doing. The software could be sending data when says it's not. Or it could be saving data in a hidden part of your operating system to be retrieved later by another malicious program. It could be doing a number of things that it shouldn't be and it would be very difficult for the end user to know.
A locally installed website, on the other hand, contains uncompiled code which is, at least somewhat, human readable. So, if you really wanted to, you could make sure that a locally installed website such as bitaddress.org isn't doing anything that it shouldn't. Although, this is infeisable for most, I once spent a while looking at brainwallet.org's source code to at least get a partial understand of how it works.