I think the OS is more important than 3rd party apps. I don't need to do 200,000 things with my tablet, I just need to browse the web, check email, hop on Google Reader, and make a Skype call. That's it. The rest is fluff that I don't need. Not everyone has the same needs, but I'm sure that 95% of people have 4 or 5 apps they use all the time, and could do just fine without a bloated app store full of garbage.
Being able to switch quickly between the 4 apps you actually use is key to a good experience. On a tablet (in particular), WebOS's superior multitasking ability will be a huge win.
I honestly think that this stuff comes down to what is most important to you. One big reason I use an Android phone, for example, is that Gmail and Google Voice are much better on it than any other mobile OS.
Tablets are the same way, it comes down to what you want to do with it and how much you want to pay.
The app selection is starting to mean less and less to me. I won't argue the iOS has a lot more quality apps, like games, but I find that the core apps that I use most are on basically every platform. I would rather get a tablet that's doing something more interesting and go beyond just a grid of icons.
Deeper is subjective. Apple's app store has a lot of redundancy and honestly, QA issues these days. I'd rather have a smaller number of quality apps that do what I need than a ton of apps to try to filter through to find the quality apps.
Also I like the ability to customize my devices a LOT more than what Apple allows on its devices and have the company say, "it's ok to play with your devices".
I vote for better base OS. Especially on a tablet where browsing the web should be just fine, why would you need an app to get access to the same information.