You can buy all the extra worlds for $2.98. I wish I could just up the price to $2.99 and not have to bother people with IAPs but the AppStore unfortunately doesn't work that way. It's incredibly hard to get people to spend $0.99 and borderline impossible to get them to spend more unless you're a big name company (I'm not). Pretty much the only way for indie devs to get sales on the AppStore is to keep the price free or $0.99 and use IAPs.
A cute little comic that explains this even better is this one: http://theoatmeal.com/blog/apps
I do know that many people don't care at all whether they end up paying 3 bucks this or either way, after they have purchased it.
Me however, I like my package all in one nicely fit together...
IAP always make me feel like... Miss out some app updates, later, waaay later come back to installing it again, trying to restore the purchases and ending up effed, because the app had been pulled (whatever reason), the purchase is not there anymore...
Now, with the all-in-one package, I'd have an outdated version, but complete... now, all that's left is IKEA furniture... You know what I mean
I simply don't feel comfortable about IAPs.
I know I'm in the minority of caring so much about preserving the integrity of my (later retro) purchases, but that's how I tick and that's all I claimed to state.
I see your dilemma and I know that I'm willing to spend more on a big brand App than on a possibly unrated indie App, however, I'd always fork out an extra buck to have it all in one package.
YES, I am paranoid.
or go with a dual strategy:
Free/Lite version + IAP
That's how many do it, that's my preferred way of making the compromises, Glassed Silver:win